debuggerd: prepare to abandon ship^Wgdb.

Talk of "gdb" when we currently mean "gdb or lldb" and will soon mean
"lldb" is starting to confuse people. Let's use the more neutral
"debugger" in places where it really doesn't matter.

The switch from gdbclient.py to lldbclient.py is a change for another
day...

Test: treehugger
Change-Id: If39ca7e1cdf4c8bb9475f1791cdaf201fbea50e0