After UserCacheDir and UserHomeDir, the only remaining piece which is
commonly needed and portable is a per-user directory to store persistent
files.

For that purpose, UserCacheDir is wrong, as it's meant only for
temporary files. UserHomeDir is also far from ideal, as that clutters
the user's home directory.

Add UserConfigDir, which is implemented in a similar manner to
UserConfigDir.

Fixes #29960.

Change-Id: I7d7a56615103cf76e2b5e2bab2029a6b09d19f0b
Reviewed-on: https://go-review.googlesource.com/c/go/+/160877
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>