OSTYPE was not being set correctly. It is in bash, not sh.
Since the value is unlikely to change, I read it once and
stored it globally
Test logic was backward in jo function, causing error to always
be printed, unless you did NOT specify a directory name.
I'm not sure if this is actually a bug in fish, but (test -d) with no argument returns true, and nonexistent environment variables return an empty string, so (test -d $XDG_DATA_HOME) returns true when $XDG_DATA_HOME isn't set.
Prior to making this change, I would get an error from dirname when opening a shell and an error from fish when cd'ing.
