GDB: introduce GNU debugger handling library

This instroduces the Avocado GDB handling library. Part of it is
built upon the gdbmi parser from the pygdb project.

Even though pygdb seems to be dead as a project, the MI language
parser is really useful and fits well with avocado's needs.

Signed-off-by: Cleber Rosa <crosa@redhat.com>
gdbtest: introduce new test

This test fires a GDB instance, sends both valid and invalid commands,
runs a couple of binaries, setting breakpoints, catching signals and
generating core dumps.

Signed-off-by: Cleber Rosa <crosa@redhat.com>