respect epa-file-encrypt-to, defaulting to it when encrypting, if set, and

respect epa-file-encrypt-to, defaulting to it when encrypting, if set, and
adjusting the value (as a file local variable and an active buffer setting)
with the result of epa-select-keys.

note the problem with caching of incorrect symmetric decryption keys.

(allout-toggle-current-subtree-encryption),
(allout-toggle-subtree-encryption): Adjust docstrings to reflect defaulting
policy change.

(allout-encrypt-string): keymod-cue rather than keypair-mode, which is
interpreted here.  Use the epa-passphrase-callback-function, in case the
user is using GnuPG v1.  Support saving of the selected keypair recipients
when invoked with a keymode-cue > 4.