rcfile: allow binding also F17...F24

For historical reasons the rebinding code allows binding F13...F16,
which on most terminals are typable with Shift plus F1...F4.  But,
at least on an Xfce Terminal, Shift plus F1...F12 produce F13...F24.
On a Linux console, Shift plus F1...F8 produce F13...F20.  So: allow
to bind the full shifted F13...F24 range.  It costs no extra code
and makes things more rounded.

doc/nano.texi

@@ -1164,8 +1164,10 @@ or the word "Space".
 Example: @code{M-C}.
 
 @item
-@code{F@var{n}} ------ where @var{n} is a numeric value from 1 to 16.
+@code{F@var{n}} ------ where @var{n} is a numeric value from 1 to 24.
 Example: @code{F10}.
+(Often, @code{F13} to @code{F24} can be typed as @code{F1} to @code{F12}
+with Shift.)
 
 @item
 @code{Ins} or @code{Del}.

doc/nanorc.5

@@ -495,8 +495,10 @@ where \fIX\fR is any ASCII character except [, or the word "Space".
 Example: M\-C.
 .TP
 .BI F N
-where \fIN\fR is a numeric value from 1 to 16.
+where \fIN\fR is a numeric value from 1 to 24.
 Example: F10.
+(Often, \fBF13\fR to \fBF24\fR can be typed as \fBF1\fR to \fBF12\fR with Shift.)
+
 .TP
 .BR Ins " or " Del .
 .RE

src/global.c

@@ -503,7 +503,7 @@ int keycode_from_string(const char *keystring)
 			return -1;
 	} else if (keystring[0] == 'F') {
 		int fn = atoi(&keystring[1]);
-		if (fn < 1 || fn > 16)
+		if (fn < 1 || fn > 24)
 			return -1;
 		return KEY_F0 + fn;
 	} else if (!strcasecmp(keystring, "Ins"))

syntax/nanorc.nanorc

@@ -11,8 +11,8 @@ icolor brightgreen "^[[:space:]]*(set|unset)[[:space:]]+(afterends|allow_insecur
 icolor 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))?\>"
 icolor brightgreen "^[[:space:]]*set[[:space:]]+(backupdir|brackets|errorcolor|functioncolor|keycolor|matchbrackets|numbercolor|operatingdir|punct|quotestr|selectedcolor|speller|statuscolor|stripecolor|titlecolor|whitespace|wordchars)[[:space:]]+"
 icolor brightgreen "^[[:space:]]*set[[:space:]]+(fill[[:space:]]+-?[[:digit:]]+|(guidestripe|tabsize)[[:space:]]+[1-9][0-9]*)\>"
-icolor brightgreen "^[[:space:]]*bind[[:space:]]+((\^([[:alpha:]]|[]@\^_]|Space)|M-([[:alpha:]]|[]!"#$%&'()*+,./0-9:;<=>?@\^_`{|}~-]|Space))|F([1-9]|1[0-6])|Ins|Del)[[:space:]]+([[:alpha:]]+|".*")[[:space:]]+(all|main|search|replace(with)?|yesno|gotoline|writeout|insert|ext(ernal)?cmd|help|spell|linter|browser|whereisfile|gotodir)([[:space:]]+#|[[:space:]]*$)"
+icolor brightgreen "^[[:space:]]*bind[[:space:]]+((\^([[:alpha:]]|[]@\^_]|Space)|M-([[:alpha:]]|[]!"#$%&'()*+,./0-9:;<=>?@\^_`{|}~-]|Space))|F([1-9]|1[0-9]|2[0-4])|Ins|Del)[[:space:]]+([[:alpha:]]+|".*")[[:space:]]+(all|main|search|replace(with)?|yesno|gotoline|writeout|insert|ext(ernal)?cmd|help|spell|linter|browser|whereisfile|gotodir)([[:space:]]+#|[[:space:]]*$)"
-icolor brightgreen "^[[:space:]]*unbind[[:space:]]+((\^([[:alpha:]]|[]@\^_]|Space)|M-([[:alpha:]]|[]!"#$%&'()*+,./0-9:;<=>?@\^_`{|}~-]|Space))|F([1-9]|1[0-6])|Ins|Del)[[:space:]]+(all|main|search|replace(with)?|yesno|gotoline|writeout|insert|ext(ernal)?cmd|help|spell|linter|browser|whereisfile|gotodir)([[:space:]]+#|[[:space:]]*$)"
+icolor brightgreen "^[[:space:]]*unbind[[:space:]]+((\^([[:alpha:]]|[]@\^_]|Space)|M-([[:alpha:]]|[]!"#$%&'()*+,./0-9:;<=>?@\^_`{|}~-]|Space))|F([1-9]|1[0-9]|2[0-4])|Ins|Del)[[:space:]]+(all|main|search|replace(with)?|yesno|gotoline|writeout|insert|ext(ernal)?cmd|help|spell|linter|browser|whereisfile|gotodir)([[:space:]]+#|[[:space:]]*$)"
 icolor brightgreen "^[[:space:]]*extendsyntax[[:space:]]+[[:alpha:]]+[[:space:]]+(i?color|header|magic|comment|linter|tabgives)[[:space:]]+.*"
 icolor brightgreen "^[[:space:]]*(syntax|linter)[[:space:]]+[^[:blank:]]+"
 icolor green "^[[:space:]]*((un)?(bind|set)|include|syntax|header|magic|comment|linter|tabgives|extendsyntax)\>"