Add poll(2), ppoll(2) and pselect(2) tests

Note that pselect(2) is disabled on FreeBSD, because is is not
allowed in capability mode.

There is also a difference in behaviour for poll(2) with
a file descriptor lacking CAP_POLL_EVENT:
 - Linux fails the entire syscall with -ENOTCAPABLE
 - FreeBSD marks that particular FD as POLLNVAL in .revents
Add POSIX message queue tests
Improve mmap(2) tests

Test case based on customcall.mmap test case from capsicum_test.c
in capsicum-linux.
Add some multi-thread tests.

These are mostly relevant for the internals of the Linux
implementation, which has internal per-thread data structures
that need to be kept in sync.
Check file pos is synced across capabilities and underlying file