Use sockets in abstract namespace only if running on linux

Also adds fallback to shell client which uses `uname -o` in place of `ostype`.

Closes #1215
Also have uname in $PATH

Travis uses Ubuntu, Ubuntu is debian-based and debian uses dash as /bin/sh, so 
$OSTYPE is not available.
.
Merge pull request #1219 from ZyX-I/limit-abstract-sockets-usage

Use sockets in abstract namespace only if running on linux