meson: Generate runstatedir according to newer standard

meson: Generate runstatedir according to newer standard

While previously FHS 2.3 defined /var/run as a place to store
runtime information [1] it's no longer 2004 and newer
specification was released which favors /run [2]. Since it was
released 10 years ago, maybe it's time we start honouring it.

On majority of Linux systems (if not all), /var/run is a symlink
to /run anyways.

Users can still pass old location via -Drunstatedir.

1: https://refspecs.linuxfoundation.org/FHS_2.3/fhs-2.3.html#VARRUNRUNTIMEVARIABLEDATA
2: https://refspecs.linuxfoundation.org/FHS_3.0/fhs/ch03s15.html

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>