options: require --magic or 'set magic' to enable the use of libmagic

Trying to determine which syntax to apply with the help of libmagic
can be a somewhat time-consuming and yet often still fruitless affair.
Therefore using libmagic should not be the default; it should require
an option to tell nano to try using libmagic when it can't determine
the applicable syntax from the file's name or first line.

This is better than having a --nomagic option (and using libmagic by
default), because when the user has 'set nomagic' in their nanorc or
--nomagic in their alias, then switching the use of libmagic back on
is troublesome.  But with --magic (and not using libmagic by default),
when nano fails to recognize a file, it is a simple ^X, add -! to the
command line, and restart.

The short form of the option is -! because we have run out of letters.

This addresses https://savannah.gnu.org/bugs/?59028.

src/color.c

@@ -189,8 +189,8 @@ void find_and_prime_applicable_syntax(void)
 	}
 
 #ifdef HAVE_LIBMAGIC
-	/* If we still don't have an answer, try using magic. */
-	if (sntx == NULL && !inhelp) {
+	/* If we still don't have an answer, try using magic (when requested). */
+	if (sntx == NULL && !inhelp && ISSET(USE_MAGIC)) {
 		struct stat fileinfo;
 		magic_t cookie = NULL;
 		const char *magicstring = NULL;

src/definitions.h

@@ -542,7 +542,8 @@ enum
 	EMPTY_LINE,
 	INDICATOR,
 	BOOKSTYLE,
-	STATEFLAGS
+	STATEFLAGS,
+	USE_MAGIC
 };
 
 /* Flags for the menus in which a given function should be present. */

src/nano.c

@@ -494,9 +494,12 @@ void usage(void)
 #endif
 	/* TRANSLATORS: The next three are column headers of the --help output. */
 	print_opt(_("Option"), _("Long option"), N_("Meaning"));
-#ifndef NANO_TINY
+#ifdef HAVE_LIBMAGIC
 	/* TRANSLATORS: The next forty or so strings are option descriptions
 	 * for the --help output.  Try to keep them at most 40 characters. */
+	print_opt("-!", "--magic", N_("Also try magic to determine syntax"));
+#endif
+#ifndef NANO_TINY
 	print_opt("-%", "--stateflags", N_("Show some states on the title bar"));
 	print_opt("-A", "--smarthome", N_("Enable smart home key"));
 	if (!ISSET(RESTRICTED)) {
@@ -1701,6 +1704,9 @@ int main(int argc, char **argv)
 		/* Whether the quoting regex was compiled successfully. */
 #endif
 	const struct option long_options[] = {
+#ifdef HAVE_LIBMAGIC
+		{"magic", 0, NULL, '!'},
+#endif
 		{"boldtext", 0, NULL, 'D'},
 #ifdef ENABLE_MULTIBUFFER
 		{"multibuffer", 0, NULL, 'F'},
@@ -1841,9 +1847,14 @@ int main(int argc, char **argv)
 	if (*(tail(argv[0])) == 'r')
 		SET(RESTRICTED);
 
-	while ((optchr = getopt_long(argc, argv, "%ABC:DEFGHIJ:KLMNOPQ:RST:UVWX:Y:Z"
+	while ((optchr = getopt_long(argc, argv, "!%ABC:DEFGHIJ:KLMNOPQ:RST:UVWX:Y:Z"
 				"abcdef:ghijklmno:pqr:s:tuvwxyz$", long_options, NULL)) != -1) {
 		switch (optchr) {
+#ifdef HAVE_LIBMAGIC
+			case '!':
+				SET(USE_MAGIC);
+				break;
+#endif
 #ifndef NANO_TINY
 			case '%':
 				SET(STATEFLAGS);

src/rcfile.c

@@ -56,6 +56,9 @@ static const rcoption rcopts[] = {
 #endif
 #ifdef ENABLE_LINENUMBERS
 	{"linenumbers", LINE_NUMBERS},
+#endif
+#ifdef HAVE_LIBMAGIC
+	{"magic", USE_MAGIC},
 #endif
 	{"morespace", MORE_SPACE},  /* Deprecated; remove in 2021. */
 #ifdef ENABLE_MOUSE