basic + fundamental: Clean up includes

Split out of #37344.

src/basic/user-util.h

@@ -7,12 +7,8 @@
 #endif
 #include <pwd.h>
 #include <shadow.h>
-#include <stdbool.h>
-#include <stdint.h>
-#include <sys/types.h>
-#include <unistd.h>
 
-#include "string-util.h"
+#include "forward.h"
 
 /* Users managed by systemd-homed. See https://systemd.io/UIDS-GIDS for details how this range fits into the rest of the world */
 #define HOME_UID_MIN ((uid_t) 60001)
@@ -43,9 +39,7 @@ const char* default_root_shell(const char *root);
 
 bool is_nologin_shell(const char *shell);
 
-static inline bool shell_is_placeholder(const char *shell) {
-        return isempty(shell) || is_nologin_shell(shell);
-}
+bool shell_is_placeholder(const char *shell) _pure_;
 
 typedef enum UserCredsFlags {
         USER_CREDS_PREFER_NSS           = 1 << 0,  /* if set, only synthesize user records if database lacks them. Normally we bypass the userdb entirely for the records we can synthesize */