Use subspawn by default for process launching

[headius]

The subspawn library is a better and cleaner implementation of Ruby's spawn (and friends) and we should try to move toward using it by default. A few things are needed for this:

• Enable the replacement spawn and popen (and anything else, like backquotes) to be the ones built on subspawn.
• Gracefully fall back on the old logic on platforms where subspawn is not supported (Windows, places we have no libfixposix build, inside a complete jar)
• Bonus: modify subspawn to still function with degraded features on regular non-libfixposix posix_spawn, similar to the old logic but using subspawn's much cleaner argument processing etc.
• Testing testing testing.