patman: Update send function to return whether it sent

patman: Update send function to return whether it sent

Indicate whether 'git send-email' was actually called, so that we don't
bother waiting for patchwork to receive our series if it wasn't.

The 'git send-email' seems to always return a code of 0 even if nothing
was sent, so we cannot use clues there. Ideally we would watch the
output to determine which patches were sent and which not, but that is
left for another day.

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