Add pdbname selection keyword.

Add pdbname selection keyword.

As a supporting change, remove trailing space from t_pdbinfo.atomnm, as
the trailing whitespace does not seem to be used anywhere.
This makes it possible to use it easily in the selection code.

Fixes #1007 in master, will backport to 4.5 branch separately.

Change-Id: Iac36bda8a84d0a6c131445e7f47ad91d7209fb10