*: build a limited version of rkt on non-Linux

This introduces build tags to allow the actual rkt binary to be built on
platforms other than Linux. This does not change our current focus of rocket
being targeted at the Linux/amd64 platform, but means that users on e.g.
OS X and other operating systems can at least use `rkt fetch` to
retrieve ACI images and so forth.
rkt: have commands register themselves
build: only build ACE validator if go linux/amd64 is available
build: Fix non-POSIX commands realpath and mktemp

realpath is not POSIX and not available on all systems.
POSIX mktemp requires a mandatory template argument.
Merge pull request #166 from jonboulle/build_osx

rkt: build a limited version on non-Linux systems