MFC r341071, r341160

r341071:
Restore the ability to override the disk unit/partition at the boot: prompt
in gptboot.

When arch-independent geli support was added, a new static 'gdsk' struct
was added, but there was still a static 'dsk' struct, and when you typed
in an alternate disk/partition, the string was parsed into that struct,
which was then never used for anything. Now the string gets parsed into
gdsk.dsk, the struct that's actually used.

r341160:
Add comments describing the bootargs handoff between loader(8) and gptboot
or zfsboot, when loader(8) is the BTX loader. No functional changes.