Improve error messages for invalid --forward-fd

[smcv]

• flatpak-spawn: Factor out parse_fd_option()

• flatpak-spawn: Validate --forward-fd arguments early

  This avoids accidentally forwarding a fd that flatpak-spawn opened for
  its own purposes, like the signalfd for forward_signals(), which would
  result in confusing error messages when flatpak-spawn tries to work
  with a fd that has already been closed.

  A frequent symptom of this is that apps that use a sandbox-escape via
  flatpak-spawn --host to mount FUSE filesystems hard-code that the
  $_FUSE_COMMFD is always going to be 3, which is not actually true
  (flatpak-spawn: Unable to read struct signalfd_siginfo: Bad file descriptor #46).

[smcv]

Now this is a proper error, instead of trampling on an unrelated fd used internally and causing weird warnings:

$ ./_build/src/flatpak-spawn --host --forward-fd=3 -- true 
./_build/src/flatpak-spawn: --forward-fd=3: Bad file descriptor
Try "./_build/src/flatpak-spawn --help" for more information.

and as a bonus, if debug messages are enabled, it will remind you that --forward-fd={0,1,2} are never necessary:

$ G_MESSAGES_DEBUG=all ./_build/src/flatpak-spawn --host --forward-fd=2 -- true
** (./_build/src/flatpak-spawn:32863): DEBUG: 13:28:48.040: Ignoring --forward-fd=2, fds 0-2 are always forwarded
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