snmp__df: restore old fieldname before rewrite.

In 19f0399dce258090c6d37410251cd2dfb520377b, get_name_by_mp() was edited to
have saner field names. Yet, changing the field names of an existing plugin is
bad as you loose history connectivity.

This commit reverts to the non-sane, but history compatible, field names.
plugins: add multigraphs support to Pgsql.pm

With the new 'multigraph' parameter it is possible to use the module in
multigraph plugin.