partial checking with substantial progress towards epg passphrase

partial checking with substantial progress towards epg passphrase
callback arrangements.  several reasons to drop the special provisions:

- gpg v1 is required for passphrase callback operation - so allout
  passphrase hinting and verification requires that
- exposes passphrase to emacs code, which is much much less secure than
  sticking with gpg v2 and leaving all passphrase handling in gpg
- leaving all passphrase handling to gpg removes a lot of complexity from
  allout code
- gpg v2 connection to gpg-agent requires no user provisions, so is simpler
  and provides some convenience that makes up for the lack of hinting and
  verification

this checkin includes a partially developed version of
allout-epg-passphrase-callback-function, with hinting and ready to
implement the passphrase verification.  but there's a lot to go there, and
in working through the twisty flow to adjust the verifier and hint string,
etc.  not worth it, considering the above trade-offs.