ipc: Use mkdtemp for more secure IPC files

Use mkdtemp makes sure that IPC files are only visible to the
owning (client) process and do not use predictable names outside
of that.

This is not meant to be the last word on the subject, it's mainly a
simple way of making the current libqb more secure. Importantly, it's
backwards compatible with an old server.

It calls rmdir on the directory created by mkdtemp way too often, but
it seems to be the only way to be sure that things get cleaned up on
the various types of server/client exit. I'm sure we can come up with
something tidier for master but I hope this, or something similar, will
be OK for 1.0.x.