proposal for an OPTION_SUBARRAY (recursive parser)

While working on OPTION_SUBARRAY I noticed a quite bad bug in how
abbrevations are dealt with, here is the fix, should be applied to next:
  [PATCH 1/4] parse-options: abbreviation engine fix.

Here is a better documentation of the struct option, only touch
comments, safe for next:
  [PATCH 2/4] Some better parse-options documentation.

Here is a 2 patch series that implement recursion and option relocation
of struct options when nested:
  [PATCH 3/4] Add OPTION_BASEOFFSET/OPTION_SUBARRAY.
  [PATCH 4/4] Implement OPTION_SUBARRAY handling.

Those two patch have nor real reason to go anywhere else than pu yet, I
post them for review and comments, and will probably base diff/log
migration on those soon.