pkgbase: differentiate package versions for ALPHA/BETA/PRERELEASE/RC phases

The current postfix conversions are:

CURRENT / STABLE / PRERELEASE, 12.x-CURRENT becomes 12.snapYYYYMMDDhhmmss
ALPHAx -> .ax, so 11.3-ALPHA1 becomes 11.3.a1.YYYYMMDDhhmmss
BETAx -> .bx, so 12.1-BETA2 becomes 12.1.b2.YYYYMMDDhhmmss
RCx -> .rcx, so 13.0-RC3 becomes 13.0.rc3.YYYYMMDDhhmmss
RELEASE -> (nothing), so 12.1-RELEASE becomes 12.1
RELEASE-pX -> pX, so 12.1-RELEASE-p1 becomes 12.1p1

Note that for development branches we will start to drop the minor version
component entirely, which more closely matches how these branches are
physically named (stable/NN).

snap is a new prefix that was added to pkg in [0], which is simply a more
verbose version of the current ".s" used.

As noted, build timestamps are also added to ALPHA/BETA/RC versions. This
is largely irrelevant for re@ snapshots because they will only produce one
set of snapshots for each alpha/beta/rc, but external folks may produce
multiple in that timeframe -- at least for alpha. For them, it is
imperative that the builds have a differentiating characteristic like this
rather than multiple builds across multiple revisions being versioned
identically.

[0] https://github.com/freebsd/pkg/pull/1929

Reviewed by: gjb, manu
Submitted by: rene (original, original version)
Differential Revision: https://reviews.freebsd.org/D28167