Differentiate package versions for ALPHA/BETA/PRERELEASE/RC phases.

Currently APLHA packages are treated as CURRENT or STABLE versions,
resulting in e.g. 13.0.s20190615125609. This version number is indeed
different from the next version number but ALPHA2 would be nicer IMO.

For the BETA, PRERELEASE and RC phases the packages are versioned the
same as for releases, so 11.3-BETA1 is 11.3 and so is 11.3-RC1, meaning
that pkg cannot easiliy upgrade from the former the next. This happened
on my Raspberry Pi which runs pkgbase.

Submitted by: rene
Approved by: manu
Event: Berlin hackathon 2019
Differential Revision: https://reviews.freebsd.org/D20651