Update pkg-stage.sh to be compatible with pkg-1.4.x.

In 1.3.x and earlier, ABI format is, for example,
freebsd:11:x86:64.

In 1.4.x, ABI format is FreeBSD:11:amd64, and a new
configuration entry, ALTABI, is: freebsd:11:x86:64.

Export PKG_ABI and PKG_ALTABI accordingly, and if
PKG_ALTABI is set, create a symlink within the dvd
PKG_CACHEDIR so both new and old ABI directories
exist.

MFC after: 3 days
Sponsored by: The FreeBSD Foundation