Switch all recvfrom and sendto socket calls to be non-blocking for higher performance and reliability (poll will still block and maintain timing)

Fix regression on socket event management that lead to deadlocks (because of blocking recvfrom use) when multiple peers listeners where used and also broke the handshake of simple profile connections

Defining htobe64/be64toh if below a minimum glibc version does not work
under musl libc where __GLIBC__ etc are not defined.

Rather than providing another logic branch for the musl case, just test
directly to see if the definitions are present.

When I try to compile against musl libc on Ubuntu 20.04, linux/if_alg.h
is not available.

This change simply uses the fallback code if the header isn't found at
configure-time.

I'm not sure if there's some missing package that would provide this.  I
searched but did not spot a candidate.

Most warnings are implicit conversions, inspecting the warnings led to finding a bug in the endian-shim where htobe32 was incorrectly defined as htons instead of htonl