Integrate tools/test/posixshm and tools/regression/posixshm into the FreeBSD
test suite as tests/sys/posixshm

Some other highlights:

• Convert the testcases over to ATF
• Don't use hardcoded paths to /tmp (which violate the ATF/kyua samdbox); use mkstemp to generate temporary paths for non-SHM_ANON shm objects.

MFC after: 2 weeks
Sponsored by: EMC / Isilon Storage Division