GDB: let user choose the GDB binary to use

Developers using Avocado's GDB support may need to use their custom
build of GDB, or simply their system has GDB at a different location.
So, allow users to optionally specify the path to the GDB binary.

Also, allow GDB API users to pass extra options to GDB via the command
line.

Changes from v1:
   * use `avocado.utils.process.find_command` as the first default
     location for the gdb binary, falling back to the hard coded
     default of `/usr/bin/gdb` if that fails.

Signed-off-by: Cleber Rosa <crosa@redhat.com>
GDB: let user choose the gdbserver binary to use

Developers using Avocado's GDB support may also need to use their custom
build of gdbserver.

This implements the same logic and options as the previous commit does
for the GDB binary, including API level changes.

Changes from v1:
   * use `avocado.utils.process.find_command` as the first default
     location for the gdbserver binary, falling back to the hard coded
     default of `/usr/bin/gdbserver` if that fails.

Signed-off-by: Cleber Rosa <crosa@redhat.com>
Merge pull request #266 from clebergnu/gdb_path_v2

GDB: support for setting gdb/gdbserver paths [v2]