Linux: UnixCmsg: Don't deduce maximum cmsg size from data size

[eddyb]

@antrik committed 10f4032:

Just because we allocate a generous receive buffer for auxilllary data,
doesn't mean that a message can't use the space for other things when it
doesn't actually carry that amount of auxillary data.

In fact, there is no reason to expect every message we receive to have
auxillary data at all -- so let's just allocate a data buffer matching
the total maximal receive size. This will enable some optimisations to
be implemented; and it simplifies the code too.

Note that the extra space is not used yet, until the sender side is
adapted to actually send larger messages making use of the extra space.
(Or even messages without auxillary data.)

This also fixes servo/servo#10260, but unlike #59, it's not just a hack.
I've put this up as a PR because @antrik might not get to it for a few days.
cc @Manishearth @pcwalton

[Manishearth]

This works for me; I'm not getting any IPC related crashes.

[antrik]

If this can wait till later today or maybe tomorrow, I'd put up a more polished PR: with a reworked commit message mentioning the bug fix; and adding a test case... It's not too important though, if this fix is urgent.

[antrik]

Please close in favour of #61