gdbserver: Reorganize linux_process_target::filter_event

Reorganize linux-low.cc:linux_process_target::filter_event such that
all the handling for events for LWPs not in the LWP list is together.
This helps make a following patch clearer.  The comments and debug
messages have also been tweaked to have them synchronized with the GDB
counterpart.

Change-Id: If9019635f63a846607cfda44b454b4254a404019