fd-util: introduce fd_reopen() helper for reopening an fd

We have the same code for this in place at various locations, let's
unify that. Also, let's repurpose test-fs-util.c as a test for this new
helper cal..

src/basic/fd-util.h

@@ -113,3 +113,5 @@ static inline int make_null_stdio(void) {
                 (fd) = -1;                      \
                 _fd_;                           \
         })
+
+int fd_reopen(int fd, int flags);