fd-util: split out fallback logic for close_all_fds()

No functional change. Just refactoring.

With this change, we can test each logic directly without seccomp
or hiding procfs.

src/basic/fd-util.h

@@ -112,6 +112,8 @@ 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 close_all_fds_by_proc(const int except[], size_t n_except);
+int close_all_fds_frugal(const int except[], size_t n_except);
 
 int pack_fds(int fds[], size_t n);