fd-util: introduce fd_validate() helper

It just uses F_GETFD to validate an fd. it's a bit easier to read
though, and handles the < 0 case internally.

src/basic/fd-util.c

@@ -511,6 +511,16 @@ int pack_fds(int fds[], size_t n_fds) {
         return 0;
 }
 
+int fd_validate(int fd) {
+        if (fd < 0)
+                return -EBADF;
+
+        if (fcntl(fd, F_GETFD) < 0)
+                return -errno;
+
+        return 0;
+}
+
 int same_fd(int a, int b) {
         struct stat sta, stb;
         pid_t pid;

src/basic/fd-util.h

@@ -80,6 +80,7 @@ int close_all_fds_without_malloc(const int except[], size_t n_except);
 
 int pack_fds(int fds[], size_t n);
 
+int fd_validate(int fd);
 int same_fd(int a, int b);
 
 void cmsg_close_all(struct msghdr *mh);