Copy stable/10@r272459 to releng/10.1 as part of
the 10.1-RELEASE process.

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

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

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

Explicitly verify if we have found the VERSION_GEN script
instead of passing the possibly null argument to awk(1) and
getting an obscure error from it.

Style: remove redundant parentheses.

Enable symbol versioning by default. Use WITHOUT_SYMVER to disable it.
Warning, after symbol versioning is enabled, going back is not easy
(use WITHOUT_SYMVER at your own risk).

Change the default thread library to libthr.

There most likely still needs to be a version bump for at least the
thread libraries. If necessary, this will happen later.

Use CPP to preprocess version map files so we can conditionalize symbols.

Break out Version.map generation code from bsd.lib.mk into a
separate bsd.symver.mk file. Include bsd.symver.mk in bsd.lib.mk
to maintain the status quo.