help: use a dedicated syntax to color shortcuts in a help text

This avoids applying the default syntax, or the syntax specified
with --syntax, to a ^G help text.

src/help.c

@@ -108,6 +108,9 @@ void do_help(void)
     int saved_margin = margin;
 	/* For avoiding the line numbers on the help screen. */
     ssize_t was_tabsize = tabsize;
+#ifndef DISABLE_COLOR
+    char *was_syntax = syntaxstr;
+#endif
     char *saved_answer = (answer != NULL) ? strdup(answer) : NULL;
 	/* Store current answer when user invokes help at the prompt. */
     unsigned stash[sizeof(flags) / sizeof(flags[0])];
@@ -173,6 +176,9 @@ void do_help(void)
 	++ptr;
     start_of_text = ptr;
 
+#ifndef DISABLE_COLOR
+    syntaxstr = "nanohelp";
+#endif
     display_the_help_text(FALSE);
     curs_set(0);
     edit_refresh();
@@ -270,6 +276,9 @@ void do_help(void)
     free(answer);
     answer = saved_answer;
     tabsize = was_tabsize;
+#ifndef DISABLE_COLOR
+    syntaxstr = was_syntax;
+#endif
 
     remove(tempfilename);
     free(tempfilename);

syntax/Makefile.am

@@ -22,6 +22,7 @@ pkgdata_DATA =	asm.nanorc \
 		man.nanorc \
 		mgp.nanorc \
 		mutt.nanorc \
+		nanohelp.nanorc \
 		nanorc.nanorc \
 		nftables.nanorc \
 		objc.nanorc \

syntax/nanohelp.nanorc

+## This is meant for highlighting key combos in a nano help text.
+
+# It should not apply to any normal file, so no fileregex.
+syntax "nanohelp"
+
+# Key combos:
+color brightblue "\^([]A-Z^\_←→↑↓]|Home|End)"  "[←→↑↓]"
+color brightblue "\<(Tab|Enter|Home|End|Ins|Del|Bsp|PgUp|PgDn)\>"
+color brightblue "\<M-."  "\<F1?[0-9]"  "(\^|M-)Space"
+
+# Quoted indicators:
+color brightred "'(\^|M-)'"