Ignore CVS directories for easier git + CVS coexistence.

ok bluhm@ stsp@

Ignore tags files

Besides obj (and all the build objects inside), tags seem to be the only
thing that's automatically created by our build infrastructure which should
never be committed.

CVS ignores "tags" due to its builtin list of ignore patterns.
Git does not ignore tags, although it has a builtin list (e.g. "*.o").
Got has no such builtin list.

Add "**/tags" to .gitignore specifically to provide a sane default for
Got checkouts.

Mirror .gitignore with sys/.gitignore to have the same experience with
kernel-only checkouts, as is common practise with Got.

OK stsp sthen

instruct git to ignore all **/obj entries created by "make obj"

ok deraadt, stsp