pw: Ensure group membership is not duplicated

Fix the following problem:

1. A nonexistent user, someuser, is added to somegroup in /etc/group.
2. someuser is then created with membership in somegroup.

The entry for somegroup in /etc/group will then contain

    somegroup:*:12345:someuser,someuser

With this fix, the entry will be

    somegroup:*:12345:someuser

PR:		238995
Reviewed by:	bapt, jrm
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D41076

(cherry picked from commit 17839f45d8)

usr.sbin/pw/pw.h

@@ -112,3 +112,5 @@ extern const char *Which[];
 
 uintmax_t strtounum(const char * __restrict, uintmax_t, uintmax_t,
     const char ** __restrict);
+
+bool grp_has_member(struct group *grp, const char *name);

usr.sbin/pw/pw_group.c

@@ -413,7 +413,7 @@ pw_group_del(int argc, char **argv, char *arg1)
 	return (EXIT_SUCCESS);
 }
 
-static bool
+bool
 grp_has_member(struct group *grp, const char *name)
 {
 	int j;

usr.sbin/pw/pw_user.c

@@ -1454,6 +1454,9 @@ pw_user_add(int argc, char **argv, char *arg1)
 	if (cmdcnf->groups != NULL) {
 		for (i = 0; i < cmdcnf->groups->sl_cur; i++) {
 			grp = GETGRNAM(cmdcnf->groups->sl_str[i]);
+			/* gr_add doesn't check if new member is already in group */
+			if (grp_has_member(grp, pwd->pw_name))
+				continue;
 			grp = gr_add(grp, pwd->pw_name);
 			/*
 			 * grp can only be NULL in 2 cases: