Fix a bug in command line option parsing code

Fix bug affecting command line options of "keyval" type. Although
escaping commands with \ is supported, it is is not applied to the
input recursively.

Signed-off-by: Nikos Skalkotos <skalkoto@grnet.gr>
Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>