syntaxes: undouble the backslash within bracket expressions

Within a bracket expression, the backslash is not special,
so it does not need to be escaped.

The double backslashes within brackets were found with:

  grep -o  '\[[^][]*\\\\[^][]*\]'  syntax{,/extra}/*rc

Also, incorporate the square brackets into some bracket expressions
by listing the closing bracket first, saving a separate regex for
those two brackets.

syntax/c.nanorc

@@ -33,7 +33,7 @@ color cyan "__attribute__[[:blank:]]*\(\([^)]*\)\)" "__(aligned|asm|builtin|hidd
 color brightyellow ""([^"]|\\")*"" "#[[:blank:]]*include[[:blank:]]*<[^>]+>"
 
 # Preprocessor directives.
-color brightcyan start="^[[:blank:]]*#[[:blank:]]*(if(n?def)?|elif|warning|error|pragma)\>" end="(\`|[^\\])$"
+color brightcyan start="^[[:blank:]]*#[[:blank:]]*(if(n?def)?|elif|warning|error|pragma)\>" end="(\`|[^\])$"
 color brightcyan "^[[:blank:]]*#[[:blank:]]*((define|else|endif|include(_next)?|line|undef)\>|$)"
 
 # Comments.

syntax/extra/gentoo.nanorc

@@ -11,7 +11,7 @@ color brightgreen "\<default(_pkg_nofetch|_src_(unpack|prepare|configure|compile
 ## Bash-related syntax:
 color green "\<(break|case|continue|do|done|elif|else|esac|exit|fi|for|function|if|in|read|return|select|shift|then|time|until|while)\>"
 color green "\<(declare|eval|exec|export|let|local)\>"
-color green "[{}():;|`$<>!=&\\]" "(\]|\[)"
+color green "[][{}():;|`$<>!=&\]"
 color green "-[defhnrsuwxzL]\>"
 color green "-(eq|ne|gt|lt|ge|le)\>"
 ## Variables... official portage ones in red, all others in bright red:

syntax/nftables.nanorc

@@ -27,7 +27,7 @@ color ,green "[[:space:]]+$"
 
 # Strings and others
 color yellow ""(\\.|[^"])*"" "'(\\.|[^'])*'"
-color green "[{}():;|`$<>!=&\\]" "(\]|\[)"
+color green "[][{}():;|`$<>!=&\]"
 
 # Basic variable names
 color brightred "(\$|@)[[:alpha:]_-][[:alnum:]_.-]*"

syntax/objc.nanorc

@@ -24,7 +24,7 @@ color magenta "[[:alnum:]]*:"
 color white "\[[^][:blank:]]*\]"
 
 # Strings.
-color brightblack "'([^'\]|(\\["'abfnrtv\\]))'" "'\\(([0-3]?[0-7]{1,2}))'" "'\\x[0-9A-Fa-f]{1,2}'"
+color brightblack "'([^'\]|(\\["'abfnrtv\]))'" "'\\(([0-3]?[0-7]{1,2}))'" "'\\x[0-9A-Fa-f]{1,2}'"
 color brightblack "<[^= 	]*>" ""(\\.|[^"])*""
 color brightblue "@"(\\.|[^"])*""
 

syntax/sh.nanorc

@@ -14,7 +14,7 @@ color brightgreen "^[A-Za-z0-9_-]+\(\)"
 # Keywords, symbols, and comparisons.
 color green "\<(break|case|continue|do|done|elif|else|esac|exit|fi|for|function|if|in|read|return|select|shift|then|time|until|while)\>"
 color green "\<(declare|eval|exec|export|let|local)\>"
-color green "[{}():;|`$<>!=&\\]" "(\]|\[)"
+color green "[][{}():;|`$<>!=&\]"
 color green "-(eq|ne|gt|lt|ge|le|ef|ot|nt)\>"
 
 # Short and long options.