Parallelize building gnu/usr.bin/groff

This speeds up building the directory from the bootstrap-tools stage in
buildworld as well as building from the subdirectory

Based on a patch submitted via -arch:
https://lists.freebsd.org/pipermail/freebsd-arch/2014-December/016493.html

MFC after: 1 week
Submitted by: Jia-Shiun Li <jiashiun@gmail.com>
Sponsored by: EMC / Isilon Storage Division
Fix sporadic build failures due to race when running make installworld
when strip gets replaced at install time by adding it to ITOOLS for the
default usr.bin/xinstall STRIP_CMD

This will fix the failure noted in this Jenkins build step:
https://jenkins.freebsd.org/job/Build-UFS-image/688/

This will also fix the issue reported by alfred@ dealing with installing on
targets that differ from build hosts (e.g. installing on i386/i386 when built
on amd64/amd64)

MFC after: 1 week
Sponsored by: EMC / Isilon Storage Division
Plug a memory leak in an error code path.

Reported by:	Coverity
CID:		1018936
MFC after: 	3 days