libsnapshot:snapuserd: Handle signals

1: poll events to accept client request.
2: Mask all signals except SIGINT and SIGTERM.

Bug: 168258493
Test: cow_snapuserd_test

Signed-off-by: Akilesh Kailash <akailash@google.com>
Change-Id: I330170ac15199d2a97cc3b752dbf3b62b7b51a40