check if valid base dir on 'buildslave upgrade-slave'

Check if we got a valid slave base dir on 'buildslave upgrade-slave'
cli command. If not, print error message and exit.
Merge branch 'upgrade-slave-chk-basedir' of git://github.com/elmirjagudin/buildbot
add Pierre to MAINTAINERS.txt
Include docs under GPLv2.

Fixes #2470.  The docs were previously under an unspecified
free-with-copyright clause, which appears to allow such relicensing.
Anyway, Warner is OK with the change.