MFH: r489253 devel/py-buildbot: Version-suffix rc script command

For python ports, particularly those that use USE_PYTHON=concurrent, only
the *default* python version/flavor (of the port) gets symlinks created for
any script/binary filenames installed in LOCALBASE/bin.

For non-default Python versions/flavors of the port or package, *only* a
version-suffixed script filename is installed.

The devel/buildbot-worker rc script was fixed in ports r483022 [1] to use a
version-suffixed script name.

This change fixes the same for devel/py-buildbot, whos rc script currently
fails to run due to trying to invoke a non-existent 'buildbot' script name
for the non-default (!2.7) case.

Since a port/package cannot know at run time, at any point what the default
Python version currently is, or whether it has changed, Python ports *must*
always and only refer to version-specific variants for everything,
including the Python interpreter, script shebangs, commands, paths, etc,
for the Python version they are currently being built with.

PR: 234565, 227675 [1]
Reported by: Andreas Sommer <andreas.sommer87 googlemail com>

Approved by: ports-secteam (miwi)