build: Reject or warn against file name length limit overruns.

* m4/guix.m4 (GUIX_TEST_ROOT_DIRECTORY, LINUX_HASH_BANG_LIMIT,
  SOCKET_FILE_NAME_LIMIT, GUIX_SOCKET_FILE_NAME_LENGTH,
  GUIX_TEST_SOCKET_FILE_NAME_LENGTH, GUIX_HASH_BANG_LENGTH,
  GUIX_CHECK_FILE_NAME_LIMITS): New macros.
* configure.ac: Use 'GUIX_CHECK_FILE_NAME_LIMITS'.
* config-daemon.ac: Use 'GUIX_TEST_ROOT_DIRECTORY'.
* test-env.in: Check socket name length and emit warning if it exceeds
  107.