basic/fd-util: refuse "infinite" loop in close_all_fds()

basic/fd-util: refuse "infinite" loop in close_all_fds()

I had a test machine with ulimit -n set to 1073741816 through pam
("session required pam_limits.so set_all", which copies the limits from PID 1,
left over from testing of #10921).

test-execute would "hang" and then fail with a timeout when running
exec-inaccessiblepaths-proc.service. It turns out that the problem was in
close_all_fds(), which would go to the fallback path of doing close()
1073741813 times. Let's just fail if we hit this case. This only matters
for cases where both /proc is inaccessible, and the *soft* limit has been
raised.

  (gdb) bt
  #0  0x00007f7e2e73fdc8 in close () from target:/lib64/libc.so.6
  #1  0x00007f7e2e42cdfd in close_nointr ()
     from target:/home/zbyszek/src/systemd-work3/build-rawhide/src/shared/libsystemd-shared-241.so
  #2  0x00007f7e2e42d525 in close_all_fds ()
     from target:/home/zbyszek/src/systemd-work3/build-rawhide/src/shared/libsystemd-shared-241.so
  #3  0x0000000000426e53 in exec_child ()
  #4  0x0000000000429578 in exec_spawn ()
  #5  0x00000000004ce1ab in service_spawn ()
  #6  0x00000000004cff77 in service_enter_start ()
  #7  0x00000000004d028f in service_enter_start_pre ()
  #8  0x00000000004d16f2 in service_start ()
  #9  0x00000000004568f4 in unit_start ()
  #10 0x0000000000416987 in test ()
  #11 0x0000000000417632 in test_exec_inaccessiblepaths ()
  #12 0x0000000000419362 in run_tests ()
  #13 0x0000000000419632 in main ()