illumos gate

In working on #850 and a related bug regarding the $PATH used by execvp() posix_spawnp() and similar, it's become desirable to have a common place for defining common paths. Such a place exists on other UNIX-like platforms, with a set of common defines for common things. <paths.h> present on BSD, OSX, and Linux.

I propose to add a <paths.h> containing those common defines which are applicable to us, and any defines of our own which become useful (likely to be the case for $PATH-based stuff, which is much more complicated for us).