Use dedicated malloc type for filecaps-related data, so we can detect any
memory leaks easier.
For some reason when I started to pass filedescent structures instead of
pointers to the file structure receiving descriptors stopped to work when also
at least few kilobytes of data is being send. In the kernel the
soreceive_generic() function doesn't see control mbuf as the first mbuf and
unp_externalize() is never called, first 6(?) kilobytes of data is missing as
well on receiving end.

This breaks for example tmux.

I don't know yet why going from 8 bytes to sizeof(struct filedescent) per
descriptor (or even to 16 bytes per descriptor) breaks things, but to
work-around it for now use 8 bytes per file descriptor at the cost of memory
allocation.

Reported by:	flo, Diane Bruce, Jan Beich <jbeich@tormail.org>
Simple testcase provided by:	mjg