Change default statedir base to /run

Currently statedir defaults to `/var/run/watchman`

This should be really changed to `/run`.

Not only would the use of `/run` be compliant with FHS (https://refspecs.linuxfoundation.org/fhs.shtml), but also on most modern Linux systems, `FHS` is a tempfs and thus automatically cleared on boot, this is unlikely to be the case with `/var/run/watchman` (e.g. `/usr/local/var/run/watchman`)

Also, in the case of `/usr/local/var/run`, this isn't even FHS compliant. You cannot expect `/usr/local/var` to exist on an FHS compliant system, let alone `/usr/local/var/run`. (https://refspecs.linuxfoundation.org/FHS_3.0/fhs/ch04s09.html)