bus: raise fd limits before dropping privs

bus: raise fd limits before dropping privs

Startup ordering was changed in #92832 to ensure that SELinux audit
messages could be sent. As a side effect, the raising of file descriptor
limits was moved to after the dropping of root privileges, resulting in
the limit change always failing.

Move the raise_file_descriptor_limit() call to ensure that it is called
before dropping root privileges.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=105165
Bug-RedHat: https://bugzilla.redhat.com/show_bug.cgi?id=1529044
[smcv: Call raise_file_descriptor_limit() even if !context->user]
Reviewed-by: Simon McVittie <smcv@collabora.com>
(cherry picked from commit 6e42964f5f850f4108fd8f7f3cd385ab4d60f9f6)