Elide passphrase caching provisions in preparation for migration from PGG

Elide passphrase caching provisions in preparation for migration from PGG
to EPA.  Doing elisions with ";;PGG " so they can be found and removed when
i'm ready to finalize the transition.

(allout-toggle-current-subtree-encryption): Mark docstring references to
passphrase cache.  (The marker is not effective as a comment within the
docstring, but they purpose is to be found and removed before release, so
tidiness is not necessary.)

Conform to changing allout-encrypt-string call signature.

(allout-encrypt-string): Remove fetch-pass argument from call signature.
Mark description for removal from docstring.

Require 'epa.

Establish not-yet-used epg-context with armoring.
Use epg-protocol as basis for the key description, instead of pgg-scheme or
pgg-default-scheme.

Elide all references to target-cache-id, and replace with value of for-key where
appropriate.  Elide all uses of fetch-pass.

Elide all passphrase cache evictions.

(allout-obtain-passphrase): Remove passphrase-cache-related parameters -
cache-id and fetch-pass - from signature and from docstring.

Use for-key instead of cache-id in pgg-read-passphrase.

Elide all passphrase cache evictions.

Adjust for allout-encrypt-string call signature changes.

(allout-create-encryption-passphrase-verifier): Adjust for
allout-encrypt-string call signature changes.

(allout-verify-passphrase): Adjust for allout-encrypt-string call signature
changes.