avocado/utils/process.py: sync SubProcess and GDBSubProcess API

The init method on SubProcess got new parameters. Even if not
implemented in GDBSubProcess, those two ducks must quack alike.

Signed-off-by: Cleber Rosa <crosa@redhat.com>
GDB: stdout support in avocado.utils.process API

Avocado's GDB support when using avocado.utils.process API
now properly populates the command result stdout attribute.

This now allows tests that run programs and check its output
(stdout only, not stderr) to work transparently under GDB.

Signed-off-by: Cleber Rosa <crosa@redhat.com>
Merge pull request #234 from clebergnu/gdb_basic_stdout

GDB stdout support