Fix issues identified by Coverity

• Always munmap memory regions after mmap'ing them.
• Make sure getpagesize() returns a value greater than 0 and use a cached value instead of always calling getpagesize(3).
• Remove intermediate variable for assigning from $TMPDIR if set in the environment to eliminate warnings about pointer conversions with "/tmp", and to mute an invalid buffer overflow concern from Coverity (snprintf and tacking on a NUL terminator was alleviating that concern before).
• Remove useless self-test of psize before it's initialized.
• Check the return values of getrlimit/setrlimit.

Cosmetic changes:

• Replace a (void*)0 with NULL.
• Do some minor whitespace clean up.
• Remove an unnecessary cast to mmap.
• Make all munmap calls use ATF_REQUIRE_MSG instead of using the:
  
  > if (munmap(..) == -1) > atf_tc_fail(..)
  
  idiom. Employ the new idiom consistently when calling munmap.

CID: 1331351, 1331382-1331386, 1331513, 1331514, 1331565, 1331583, 1331694
Differential Revision: https://reviews.freebsd.org/D6012
MFC after: 2 weeks
Reported by: Coverity
Reviewed by: markj
Sponsored by: EMC / Isilon Storage Division