bitcoind: Daemonize using daemon(3)

Simplified version of #8278. Assumes that every OS that (a) is supported
by Bitcoin Core (b) supports daemonization has the daemon() function
in its C library.

• Removes the fallback path for operating systems that support daemonization but not daemon(). This prevents never-exercised code from ending up in the repository (see discussion here: https://github.com/bitcoin/bitcoin/pull/8278#issuecomment-242704745).

• Removes the windows-specific path. Windows doesn't support daemon(), so it don't support daemonization there, automatically.

Original code by Matthew King, adapted by Wladimir van der Laan.