fd-util: Expose helper to pack fds into 3,4,5,...

This is useful for situations where an array of FDs is to be passed into
a child process (i.e. by passing it through safe_fork). This function
can be called in the child (before calling exec) to pack the FDs to all
be next to each-other starting from SD_LISTEN_FDS_START (i.e. 3)

src/basic/fd-util.h

@@ -77,6 +77,8 @@ int get_max_fd(void);
 int close_all_fds(const int except[], size_t n_except);
 int close_all_fds_without_malloc(const int except[], size_t n_except);
 
+int pack_fds(int fds[], size_t n);
+
 int same_fd(int a, int b);
 
 void cmsg_close_all(struct msghdr *mh);