su.c: implement --exec-command

It's now possible to run commands as other users without shell
interpolation by using "--exec-command":

Read /etc/shadow as root without specifying user:
```
su --exec-command /bin/cat -- /etc/shadow
```

Or specify user:
```
su --exec-command /bin/cat root -- /etc/shadow
```

src/su.c

@@ -94,6 +94,7 @@ static bool do_interactive_shell = false;
 static bool fakelogin = false;
 static /*@observer@*/const char *shellstr;
 static /*@null@*/char *command = NULL;
+static /*@null@*/char *exec_command = NULL;
 static int optidx;
 
 
@@ -440,12 +441,14 @@ static void usage (int status)
 	         "\n"
 	         "Options:\n"
 	         "  -c, --command COMMAND         pass COMMAND to the invoked shell\n"
+	         "  -e, --exec PATH               run PATH without shell, follow -- with args\n"
 	         "  -h, --help                    display this help message and exit\n"
 	         "  -, -l, --login                make the shell a login shell\n"
 	         "  -m, -p,\n"
 	         "  --preserve-environment        do not reset environment variables, and\n"
 	         "                                keep the same shell\n"
 	         "  -s, --shell SHELL             use SHELL instead of the default in passwd\n"
+	         "  --                            pass all subsequent arguments on as-is\n"
 	         "\n"
 	         "If no username is given, assume root.\n"), (E_SUCCESS != status) ? stderr : stdout);
 	exit (status);
@@ -820,6 +823,12 @@ static void process_flags (int argc, char **argv)
 			}
 
 			command = argv[++optidx];
+		} else if (flags_match (arg, "--exec-command", "-e", NULL)) {
+			if (optidx == argc - 1) {
+				flag_arg_required (arg);
+			}
+
+			exec_command = argv[++optidx];
 		} else if (flags_match (arg, "--help", "-h", NULL)) {
 			usage (E_SUCCESS);
 		} else if (flags_match (arg, "--login", "-l", "-")) {
@@ -843,6 +852,17 @@ static void process_flags (int argc, char **argv)
 		}
 	}
 
+	if (NULL != exec_command && NULL != command) {
+		fprintf (stderr,
+		         _("%s: COMMAND and PATH are mutually exclusive\n"),
+		         argv[0]);
+		usage (E_USAGE);
+	}
+
+	if (NULL != exec_command) {
+		command = exec_command;
+	}
+
 	/* if next arg is not "--", treat as USER */
 	if (optidx < argc && strcmp (argv[optidx], "--")) {
 		STRFCPY (name, argv[optidx++]);	/* use this login id */
@@ -1226,10 +1246,18 @@ int main (int argc, char **argv)
 		 * with the rest of the command line included.
 		 */
 		argv[-1] = cp;
-		execve_shell (shellstr, &argv[-1], environ);
-		err = errno;
-		(void) fprintf (stderr,
-		                _("Cannot execute %s\n"), shellstr);
+
+		if (NULL != exec_command) {
+			(void) execve (command, &argv[1], environ);
+			err = errno;
+			(void) fprintf (stderr,
+					_("Cannot execute \'%s\'\n"), command);
+		} else {
+			execve_shell (shellstr, &argv[-1], environ);
+			err = errno;
+			(void) fprintf (stderr,
+					_("Cannot execute \'%s\'\n"), shellstr);
+		}
 		errno = err;
 	} else {
 		(void) shell (shellstr, cp, environ);