pkgbase: differentiate package versions for ALPHA/BETA/PRERELEASE/RC phases (part 2)

[rene]

Summary

https://reviews.freebsd.org/D20651 had some mistakes, fix these with an updated patch.

The current postfix conversions are:

• CURRENT / STABLE -> .sYYYYMMDDhhmmss
• ALPHAx -> .ax, so 11.3-ALPHA1 becomes 11.3.a1
• BETAx -> .bx, so 12.1-BETA2 becomes 12.1.b2
• RCx -> .rx, so 13.0-RC3 becomes 13.0.r3
• PRERELEASE -> .p, so 11.3-PRERELEASE becomes 11.3.p
• RELEASE -> (nothing), so 12.1-RELEASE becomes 12.1

For any -pX, a package revision of X is added, so 12.1-BETA2-p3 becomes 12.1.b2_3

Test Plan

Tested with local script and building 11.3.r1 packages

[mat]

This is quite similar to D8943.

[rene]

Merge ideas from D8943

[rene]

Remove the ".0" in version numbers for FreeBSD-CURRENT (so 13.s20190919151659 instead of 13.0.s20190919151659). Taken from D8943