Add kern.geom.part.separator tunable. This makes it possible
to specify an optional separator to insert before partition name;
eg if it's set to "c/", you'll get "ada0c/s1" instead of "ada0s1".
(It cannot be set to just “/“, since ada0 is a device node, not
a directory.)

Reviewed by: imp
MFC after: 2 weeks
Sponsored by: Klara Inc.
Differential Revision: https://reviews.freebsd.org/D22193