fix the test for getdtablecount The configure ran it but wasn't present in the tests.c file. Will need to switch away from this all-in-one someday...

switch from libevent to a smaller, thin wrapper around poll() libevent is a very cool library, I like its APIs and enjoy using it. However, for amused it is maybe a bit too much since it doesn't deal with too many file descriptors. Amused only needs to monitor one fd for the player process and the currently connected clients. Given that it runs per-user locally, having more than a dozen of fds open is very, very rare.

configure: allow to select the audio backend

fix sndio detection the compiler may complain that there's no return: tests.c: In function 'main': tests.c:954: warning: control reaches end of non-void function spotted by the OpenBSD bulk builds on sparc64 (using gcc 4.2)

fix landlock detection apparently it's not always possible to include both linux/prctl.h and sys/prctl.h.

use (a modified version of) kristaps' oconfigure see GitHub issue #1