basic/fd-util: sort the 'except' array in place

We need a sorted list of fds to skip over when closing. We would allocate a
copy of the passed array to do the sort. But all callers construct a temporary
array to pass to us, so it is pointless to copy it again.

close_all_fds/safe_fork_full/namespace_fork/fork_agent are changed to pass
a non-const int array. I checked all users, and all callers are fine with
the array being sorted.

The function was returning some number (sometimes 1, sometimes the extent
of the range passed over to close_range(), ???). Anyway, all callers only
check for error, so let's return 0 on success.

src/basic/process-util.h

@@ -168,15 +168,15 @@ typedef enum ForkFlags {
         FORK_NEW_USERNS         = 1 << 13, /* Run child in its own user namespace */
 } ForkFlags;
 
-int safe_fork_full(const char *name, const int except_fds[], size_t n_except_fds, ForkFlags flags, pid_t *ret_pid);
+int safe_fork_full(const char *name, int except_fds[], size_t n_except_fds, ForkFlags flags, pid_t *ret_pid);
 
 static inline int safe_fork(const char *name, ForkFlags flags, pid_t *ret_pid) {
         return safe_fork_full(name, NULL, 0, flags, ret_pid);
 }
 
-int namespace_fork(const char *outer_name, const char *inner_name, const int except_fds[], size_t n_except_fds, ForkFlags flags, int pidns_fd, int mntns_fd, int netns_fd, int userns_fd, int root_fd, pid_t *ret_pid);
+int namespace_fork(const char *outer_name, const char *inner_name, int except_fds[], size_t n_except_fds, ForkFlags flags, int pidns_fd, int mntns_fd, int netns_fd, int userns_fd, int root_fd, pid_t *ret_pid);
 
-int fork_agent(const char *name, const int except[], size_t n_except, pid_t *pid, const char *path, ...) _sentinel_;
+int fork_agent(const char *name, int except[], size_t n_except, pid_t *pid, const char *path, ...) _sentinel_;
 
 int set_oom_score_adjust(int value);