Replace strncpy() with strlcpy() when parsing login time limit strings

from /etc/login.conf, or an unterminated string buffer could result.
Probably, login_times.c should reject excessively long time strings as
unparseable, rather than truncating, which might render an invalid
string valid.

Found with:	Coverity Prevent (tm)
Reviewed by:	csjp
MFC after:	3 days

lib/libutil/login_times.c

@@ -72,7 +72,7 @@ parse_lt(const char * str)
 	char		buf[64];
 
 	/* Make local copy and force lowercase to simplify parsing */
-	p = strncpy(buf, str, sizeof buf);
+	p = strlcpy(buf, str, sizeof buf);
 	buf[sizeof buf - 1] = '\0';
 	for (i = 0; buf[i]; i++)
 	    buf[i] = (char)tolower(buf[i]);