Add DragonFly BSD support

Port RVM to DragonFly <http://dragonflybsd.org>, a FreeBSD fork.

Dependency installation only supports binary packages via pkgng, neither
building from source nor pkgsrc are implemented.
fix "bad math expression"
Merge pull request #2063 from matthiasr/dragonfly-support

Add Dragonfly support
handle spaces and slashes in log file names, update #2051