Initial implementation of multi mysql support.

The aim is to support multiple versions of MySQL so that utilities are
not tied to the specific version they happen to have been built against,
primarily so that the percona-toolkit package can be version agnostic.

The main entry point is p5-DBD-mysql, and this is updated to have
multiple versions built.  Packages will need to be updated to depend
upon all possible combinations of the p5-DBD-mysql package.
Rewrite mysql.buildlink3.mk.

This brings it into line with the simpler apache.mk, and adds support
for databases/mysql56-client, as well as fixing some bugs with the
previous implementation.
Update to MYSQL_VERSION from MYSQL_PKG_PREFIX.
Fix variable name.
Update MySQL multi var.
mysql50 is no longer valid.
testing..
Go back to MYSQL_PKG_PREFIX.

We need to differentiate between MYSQL_VERSION and MYSQL_PKG_PREFIX so
that we can retain support for the legacy MYSQL_VERSION_DEFAULT.
Syntax fix for previous.
Convert remaining p5-DBD-mysql dependencies.
Merge branch 'joyent/feature/pbulkmulti/2013Q1' into joyent/release/2013Q1
Update 'wip' submodule.