Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Backout libinstall.a -> libpkg commit.

Discussed with: erwin, brooks, bapt

Introduce notion of the package tools revision and allow to wrap all tools
included into pkg_install according to the content of /var/db/pkg_install.conf
file, which specifies version and alternative location of the tools. Format
of the said file is very simple: one line which specifies revision of the
alternative version of the tools and their location separated by space,
i.e.:

20030102 /usr/local/sbin

This would allow bsd.port.mk to install and use up to date version of tools
on older system from ports.

Also add new `-P' flag to pkg_info, which causes it to report currently
installed version of package tools.

Discussed with: will