Fix issues identified by Coverity

[ngie]

Summary

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.

CID: 1331351, 1331382-1331386, 1331513, 1331514, 1331565, 1331583, 1331694
MFC after: 2 weeks
Reported by: Coverity
Sponsored by: EMC / Isilon Storage Division

Test Plan

sudo kyua test -k /usr/tests/sys/posixshm

[ngie]

lib/libc/gen/getpagesize.c

56

Ignore this part; I'm going to commit this separately (I wish arc diff --create didn't grab everything in one's workspace..)

[ngie]

Remove getpagesize.c portion that accidentally snuck into the previous diff

[markj]

Looks ok to me.

tests/sys/posixshm/posixshm_test.c

174

In some places the return value of munmap() is checked, and in others it isn't.

[ngie]

tests/sys/posixshm/posixshm_test.c

174

Fixed by using ATF_REQUIRE_MSG consistently. The old idiom was a bit awkward..