execute: move suppression of HOME=/ and SHELL=/bin/nologin into user-util.c

This adds a new call get_user_creds_clean(), which is just like
get_user_creds() but returns NULL in the home/shell parameters if they contain
no useful information. This code previously lived in execute.c, but by
generalizing this we can reuse it in run.c.

src/basic/user-util.c

@@ -31,14 +31,15 @@
 #include <unistd.h>
 #include <utmp.h>
 
-#include "missing.h"
 #include "alloc-util.h"
 #include "fd-util.h"
 #include "formats-util.h"
 #include "macro.h"
+#include "missing.h"
 #include "parse-util.h"
 #include "path-util.h"
 #include "string-util.h"
+#include "strv.h"
 #include "user-util.h"
 #include "utf8.h"
 
@@ -175,6 +176,35 @@ int get_user_creds(
         return 0;
 }
 
+int get_user_creds_clean(
+                const char **username,
+                uid_t *uid, gid_t *gid,
+                const char **home,
+                const char **shell) {
+
+        int r;
+
+        /* Like get_user_creds(), but resets home/shell to NULL if they don't contain anything relevant. */
+
+        r = get_user_creds(username, uid, gid, home, shell);
+        if (r < 0)
+                return r;
+
+        if (shell &&
+            (isempty(*shell) || PATH_IN_SET(*shell,
+                                            "/bin/nologin",
+                                            "/sbin/nologin",
+                                            "/usr/bin/nologin",
+                                            "/usr/sbin/nologin")))
+                *shell = NULL;
+
+        if (home &&
+            (isempty(*home) || path_equal(*home, "/")))
+                *home = NULL;
+
+        return 0;
+}
+
 int get_group_creds(const char **groupname, gid_t *gid) {
         struct group *g;
         gid_t id;

src/basic/user-util.h

@@ -40,6 +40,7 @@ char* getlogname_malloc(void);
 char* getusername_malloc(void);
 
 int get_user_creds(const char **username, uid_t *uid, gid_t *gid, const char **home, const char **shell);
+int get_user_creds_clean(const char **username, uid_t *uid, gid_t *gid, const char **home, const char **shell);
 int get_group_creds(const char **groupname, gid_t *gid);
 
 char* uid_to_name(uid_t uid);