Add accept4 fallback
Add accept4 fallback

(cherry picked from commit e84b49ad79f734b1b21daa9d848e74158f03717d)

# Conflicts:
#	socket_header.h

uroni/urbackup_backend,uroni/urbackup_backend,uroni/urbackup_backend,uroni/urbackup_backend,uroni/urbackup_backend,uroni/urbackup_backend,uroni/urbackup_backend