patman: Refactor how the default subcommand works

At present patman tries to assume a default subcommand of 'send', to
maintain backwards compatibility. However it does not cope with
arguments added to the default command, so for example 'patman -t'
does not work.

Update the logic to handle this. Also update the CC command to use 'send'
explicitly, since otherwise patman gets confused with the patch-filename
argument.

Signed-off-by: Simon Glass <sjg@chromium.org>