cleanup: unify common expressions in cbgetline

Using exactly the same expressions makes code review easier (you only
have to think about them once instead of twice).
Unify return codes for cbgetline and cbgetraw

It may make little sense to search for a line feed when reading raw data,
but this makes the return code actually useful across calls to cbgetline
and cbgetraw (you can tell whether cbgetline needs more data).
Introduce getline_buffered() to get the number of buffered bytes

This function gets the number of bytes "in the queue". It also makes
the meaning of the condition in cbgetline and cbgetraw easier to grasp.
Fix untagged responses

The tagged argument is now ignored.
Merge branch 'master' of github.com:ptesarik/crashgui