rcfile: rename 'extcmd' to 'execute', to be more readable and fitting

Also, improve its description in the documentation.

doc/nano.texi

@@ -1570,9 +1570,6 @@ The 'write file' menu.
 @item insert
 The 'insert file' menu.
 
-@item extcmd
-The menu for inserting output from an external command, reached from the insert menu.
-
 @item browser
 The file browser, for choosing a file to read from or write to.
 
@@ -1582,6 +1579,12 @@ The 'search for a file' menu in the file browser.
 @item gotodir
 The 'go to directory' menu in the file browser.
 
+@item execute
+The menu for inserting the output from an external command,
+or for filtering the buffer (or the marked region) through
+an external command, or for executing one of several tools.
+(The old form of this menu name, 'extcmd', is deprecated.)
+
 @item spell
 The menu of the integrated spell checker where the user can edit a misspelled word.
 

doc/nanorc.5

@@ -888,9 +888,6 @@ The 'write file' menu.
 .B insert
 The 'insert file' menu.
 .TP
-.B extcmd
-The menu for inserting output from an external command, reached from the insert menu.
-.TP
 .B browser
 The file browser for inserting or writing a file.
 .TP
@@ -900,6 +897,12 @@ The 'search for a file' menu in the file browser.
 .B gotodir
 The 'go to directory' menu in the file browser.
 .TP
+.B execute
+The menu for inserting the output from an external command,
+or for filtering the buffer (or the marked region) through
+an external command, or for executing one of several tools.
+(The old form of this menu name, 'extcmd', is deprecated.)
+.TP
 .B spell
 The menu of the integrated spell checker where the user can edit a misspelled word.
 .TP

src/rcfile.c

@@ -149,15 +149,15 @@ static colortype *lastcolor = NULL;
 		/* The end of the color list for the current syntax. */
 #endif
 
-#define NUMBER_OF_MENUS  16
+#define NUMBER_OF_MENUS  17  /* Remove the deprecated 'extcmd' in 2022. */
 char *menunames[NUMBER_OF_MENUS] = { "main", "search", "replace", "replacewith",
 									"yesno", "gotoline", "writeout", "insert",
-									"extcmd", "help", "spell", "linter",
+									"execute", "extcmd", "help", "spell", "linter",
 									"browser", "whereisfile", "gotodir",
 									"all" };
 int menusymbols[NUMBER_OF_MENUS] = { MMAIN, MWHEREIS, MREPLACE, MREPLACEWITH,
 									MYESNO, MGOTOLINE, MWRITEFILE, MINSERTFILE,
-									MEXECUTE, MHELP, MSPELL, MLINTER,
+									MEXECUTE, MEXECUTE, MHELP, MSPELL, MLINTER,
 									MBROWSER, MWHEREISFILE, MGOTODIR,
 									MMOST|MBROWSER|MHELP|MYESNO };
 #endif /* ENABLE_NANORC */

syntax/nanorc.nanorc

@@ -11,7 +11,7 @@ color brightgreen "^[[:space:]]*(set|unset)[[:space:]]+(afterends|allow_insecure
 color yellow "^[[:space:]]*set[[:space:]]+((error|function|key|number|selected|status|stripe|title)color)[[:space:]]+(bright)?(white|black|red|blue|green|yellow|magenta|cyan|normal)?(,(white|black|red|blue|green|yellow|magenta|cyan|normal))?\>"
 color brightgreen "^[[:space:]]*set[[:space:]]+(backupdir|brackets|errorcolor|functioncolor|keycolor|matchbrackets|numbercolor|operatingdir|punct|quotestr|selectedcolor|speller|statuscolor|stripecolor|titlecolor|whitespace|wordchars)[[:space:]]+"
 color brightgreen "^[[:space:]]*set[[:space:]]+(fill[[:space:]]+-?[[:digit:]]+|(guidestripe|tabsize)[[:space:]]+[1-9][0-9]*)\>"
-color brightgreen "^[[:space:]]*bind[[:space:]]+((\^([A-Za-z]|[]/@\^_`]|Space)|([Ss][Hh]-)?[Mm]-[A-Za-z]|[Mm]-([]!"#$%&'()*+,./0-9:;<=>?@\^_`{|}~-]|Space))|F([1-9]|1[0-9]|2[0-4])|Ins|Del)[[:space:]]+([a-z]+|".*")[[:space:]]+(all|main|search|replace(with)?|yesno|gotoline|writeout|insert|ext(ernal)?cmd|help|spell|linter|browser|whereisfile|gotodir)([[:space:]]+#|[[:space:]]*$)"
+color brightgreen "^[[:space:]]*bind[[:space:]]+((\^([A-Za-z]|[]/@\^_`]|Space)|([Ss][Hh]-)?[Mm]-[A-Za-z]|[Mm]-([]!"#$%&'()*+,./0-9:;<=>?@\^_`{|}~-]|Space))|F([1-9]|1[0-9]|2[0-4])|Ins|Del)[[:space:]]+([a-z]+|".*")[[:space:]]+(main|help|search|replace(with)?|yesno|gotoline|writeout|insert|browser|whereisfile|gotodir|execute|spell|linter|all)([[:space:]]+#|[[:space:]]*$)"
 color brightgreen "^[[:space:]]*unbind[[:space:]]+((\^([A-Za-z]|[]/@\^_`]|Space)|([Ss][Hh]-)?[Mm]-[A-Za-z]|[Mm]-([]!"#$%&'()*+,./0-9:;<=>?@\^_`{|}~-]|Space))|F([1-9]|1[0-9]|2[0-4])|Ins|Del)[[:space:]]+(all|main|search|replace(with)?|yesno|gotoline|writeout|insert|ext(ernal)?cmd|help|spell|linter|browser|whereisfile|gotodir)([[:space:]]+#|[[:space:]]*$)"
 color brightgreen "^[[:space:]]*extendsyntax[[:space:]]+[[:alpha:]]+[[:space:]]+(i?color|header|magic|comment|formatter|linter|tabgives)[[:space:]]+.*"
 color brightgreen "^[[:space:]]*(syntax[[:space:]]+[^[:blank:]]+|(formatter|linter)[[:space:]]+.+)"