Rework build to support more liberal unix variants.

Rather than having the build setup such that it will only work on the
specifically defined operating systems, this commit modifies it to use
more liberal !windows,!plan9 build tag for the unix specific bits.

This means bolt will compile on more Operating Systems such as OpenBSD,
FreeBSD, and NetBSD.

See boltdb/bolt#257 for discussion.
Merge pull request #258 from davecgh/davec_build

Rework build to support more liberal unix variants.