Fix leaks and test for getpagesize() returning == -1

• close file descriptors after use.
• 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).

CID: 1331374-1331377, 1331653-1331662
Differential Revision: https://reviews.freebsd.org/D6011
MFC after: 2 weeks
Reported by: Coverity
Reviewed by: cem
Sponsored by: EMC / Isilon Storage Division