zalloc_malloc:Free hexdump preceeding buffer when we detect overflow

Move hexdump from stand/common/misc.c to stand/libsa/hexdump.c
(svn cp)
Disable use of pager - causes linking issue for boot1
can be re-enabled by defining HEXDUMP_PAGER.

Reviewed by: stevek, imp
MFC after: 1 week
Sponsored by: Juniper Networks
Differential Revision: https://reviews.freebsd.org/D26235