fix get_user_choice_multiple_sides

the comment in replay.cpp says we want "send data to others" so
calling "pull_remote_user_input" is not really correct. I added a
function synced_context::send_user_choice for this case.

this fixes a bug where players can get an oos error in
wesnoth.synconize_choice
I backported a simpler version to 1.12:
0e79ef919f5718cd8024bbbac31ed1e7b67e4fc7 because i don't know whether
this bug can happen in 1.12 too.