libbb: introduce and use xgetpwnam. ~ -150 bytes.

diff --git a/include/libbb.h b/include/libbb.h
index a34e8a1..80311db 100644
--- a/include/libbb.h
+++ b/include/libbb.h
@@ -702,6 +702,7 @@
 void xget_uidgid(struct bb_uidgid_t*, const char*) FAST_FUNC;
 /* chown-like handling of "user[:[group]" */
 void parse_chown_usergroup_or_die(struct bb_uidgid_t *u, char *user_group) FAST_FUNC;
+struct passwd* xgetpwnam(const char *name) FAST_FUNC;
 struct passwd* xgetpwuid(uid_t uid) FAST_FUNC;
 struct group* xgetgrgid(gid_t gid) FAST_FUNC;
 char* xuid2uname(uid_t uid) FAST_FUNC;