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

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

Remove long-unused GNU ar and ranlib

The libarchive-based replacements have been used since 2009; the GNU
ones were kept to support source upgrades from FreeBSD 6.

Approved by: re@ (delphij)

Regenerate src.conf(5) for mdoc(7) and typo corrections.

PR: 167804
Submitted by: Nobuyuki Koganemaru (kogane!jp.freebsd.org)
MFC after: 3 days

Implement WITH{,OUT}_{GCC,BINUTILS} to provide finer-grained control
over building gcc and binutils. They default to true, unless
MK_TOOLCHAIN is no.

Reviewed by: ru@