Stop using $USER for shadow entries

This was extremely bad practice, effectively making the program behave
different depending on which architecture you are running it on.

OpenBSD offers getpwuid_shadow, but there is no getspuid for getspnam,
so we resort to using the pw_name entry in the struct passwd we filled
earlier.

This prevents slock from crashing when $USER is empty (easy to do). If
you want to run slock as a different user, don't use

	$ USER="tom" slock

but doas or sudo which were designed for this purpose.

slock.c

@@ -103,14 +103,14 @@ gethash(void)
 #if HAVE_SHADOW_H
 	if (hash[0] == 'x' && hash[1] == '\0') {
 		struct spwd *sp;
-		if (!(sp = getspnam(getenv("USER"))))
+		if (!(sp = getspnam(pw->pw_name)))
 			die("slock: getspnam: cannot retrieve shadow entry (make sure to suid or sgid slock)\n");
 		hash = sp->sp_pwdp;
 	}
 #else
 	if (hash[0] == '*' && hash[1] == '\0') {
 #ifdef __OpenBSD__
-		if (!(pw = getpwnam_shadow(getenv("USER"))))
+		if (!(pw = getpwuid_shadow(getuid())))
 			die("slock: getpwnam_shadow: cannot retrieve shadow entry (make sure to suid or sgid slock)\n");
 		hash = pw->pw_passwd;
 #else