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>