Add a bit more debug messages.
Rework bootparttest to use more code from sys/boot.
Use disk_open() call to emulate loader behavior.
Fix typo.
Eliminate use of M_EXT in IP6_EXTHDR_CHECK() by trimming a redundant
'if'/'else' case: it matches the simple 'else' case that follows.

This reduces awareness of external-storage mechanics outside of the
mbuf allocator.

Reviewed by:	bz
MFC after:	3 days
Sponsored by:	EMC / Isilon Storage Division
Differential Revision:	https://reviews.freebsd.org/D900
Avoid unnecessary ppeers_lock acquisition in exit1.

MFC after:	1 week
- Refactor defining variables.
- Merge common modules both i386 and amd64 into one if-endif.
- Sort.
- There are no functional changes.
1) For %Z format, understand "UTC" name too.
2) Return NULL if timegm() fails, because it means we can convert
what we have in GMT to local time needed.