Tag filters like `-tagfilter=foo=1` currently cannot be used to filter
by a unitless numeric tag as the tag filter regular expression expects
the unit part to be always present.  They can be made working by using
`-tagfilter=foo=1unit` instead, but that's weird syntax. So fix this by
merely making the unit part optional.

I was wondering if this simple fix could have any unintended effects but
as far as I can tell from testing it does not.