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

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

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

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

Add src.conf option WITH_SHARED_TOOLCHAIN to enable building the
toolchain binaries as dynamically linked. Option is disabled by
default.

Reviewed by: ru (previous version)
MFC after: 2 weeks