syscall_server: fix epoll_wait implementation

[Officeyutong]

This PR fixes the implementation of epoll_wait in syscall server, so make bpftrace exit properly.

In kernel, syscall epoll_wait could be interrupted by signal, when either no requests were received, or the timeout expired.

In the previous epoll_wait of syscall-server, we doesn't implement similar things. So when signals were devilered during the execution of epoll_wait, they were handled normally (e.g directly call the signal handler).

In the current implementation, we block the develivery of SIGINT and SIGTERM (more could be added after suitable tests) during the execution of epoll_wait, but unblock them when epoll_wait is sleeping. epoll_wait will use sigtimedwait to wait for SIGINT and SIGTERM when no requests were received. Once these signal were develivered when sleeping, we just set errno to EINTR, and return -1 from epoll_wait. This will keep the same behavior like the kernel epoll_wait implementation.

Closes #99