unix: emulate sendfile if necessary
unix: emulate sendfile if necessary

Some platforms don't support sendfile() (netbsd, openbsd), others don't support
arbitrary file descriptors (freebsd, darwin) and yet others have quirks in their
implementation (sunos).

Work around the quirks. If all else fails, fall back to sendfile() emulation.

urbit/archaeology2,urbit/archaeology2,urbit/archaeology2,unknownbrackets/maxcso,urbit/archaeology2,urbit/archaeology2,unknownbrackets/maxcso,unknownbrackets/maxcso,urbit/archaeology2,urbit/archaeology2,unknownbrackets/maxcso,urbit/archaeology2,unknownbrackets/maxcso,unknownbrackets/maxcso,urbit/archaeology2,unknownbrackets/maxcso