serial: extend semantics of connection and comm params extraction helper

The sr_serial_extract_options() routine is a hidden gem, has existed for
almost eight years, and is exclusively used by the motech-lps-301 driver
(was also used by hameg-hmo at some point in the past). Being so useful
and doing something to terribly popular, more drivers should use it.

Extend the API to make either of the out parameters genuinely optional,
and guarantee that pre-assigned defaults will survive in the absence of
user specs. Update the Doxygen comment.