Add support for adding empty partition entries. I.e. skip partition
numbers or names. This gives more control over the actual layout and
helps to construct BSD disklabels with /usr or /var at dedicated
partitions.

Obtained from:	Juniper Networks, Inc.
MFC after:	3 days
Relnotes:	yes
Fix duplicate PKG_ABI in the PKG_CACHEDIR path.

MFC after:	3 days
X-MFC-With:	r271480
Sponsored by:	The FreeBSD Foundation