util-lib: move take_password_lock() to user-util.[ch]

Also, rename it take_etc_passwd_lock(), in order to make it more
expressive.

src/firstboot/firstboot.c

@@ -38,6 +38,7 @@
 #include "strv.h"
 #include "terminal-util.h"
 #include "time-util.h"
+#include "user-util.h"
 
 static char *arg_root = NULL;
 static char *arg_locale = NULL;  /* $LANG */
@@ -536,7 +537,7 @@ static int process_root_password(void) {
 
         mkdir_parents(etc_shadow, 0755);
 
-        lock = take_password_lock(arg_root);
+        lock = take_etc_passwd_lock(arg_root);
         if (lock < 0)
                 return lock;