Set default maximum number of Unix fds according to OS

Set default maximum number of Unix fds according to OS

QNX has an arbitrary limit to the number of file descriptors
which may be passed in a message, which is smaller than the
current default.  This patch therefore changes the default from
a hardcoded constant to a macro, which is determined at configure
time by looking at the host operating system.

[This reduces the limit from 4096 (session)/1024 (system) to 128 fds
per message on QNX, and 1024 fds per message on other operating systems.
I think the reduced session bus limit on other OSs is a reasonable change
too, given that the default hard/soft ulimits in Linux are only 4096/1024
fds per process. -smcv]

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=61176
Reviewed-by: Simon McVittie <simon.mcvittie.collabora.co.uk>