bash login shell invocation in mock

[Paul Howarth]

mock now uses a login shell for the build phase, invoking bash as:

   bash -l -c ...

It would be nice if this was changed to:

   bash --login -c ...

(twi lines need changing in backend.py)

This is because the --login long option pre-dates the -l short option in 
bash; the short version isn't understood in some really old 
distributions (like RHL7, don't ask!) and its use precludes use of mock 
in building packages for those releases. The long version seems to work 
everywhere.

Paul.