listrunner: convert from getopt to optparse

The “-A” (use agent) was not documented, and instead of adding manual
listing, I converted it to optparse like the other CLI tools.

Note that I cleaned up a bit the usage and help texts.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>