Implement POSIX network redirects

[acw]

Write the backend functions for musl that deal with networking; I believe they'll all need to start with syscall_SYS_ or socketcall. These should either (1) check for an implementation provided by the user via the base HaLVM libraries and dispatch to them if they exist, or (2) just dispatch into the HaLVM, because we're going to implement default behavior in Haskell, anyways.

I suspect that while version 2 is nicer, version 1 will be necessary to avoid infinite loops in the system.