(pgg-decrypt): Passing along 'passphrase' in call to pgg-decrypt-region.

(pgg-decrypt): Passing along 'passphrase' in call to pgg-decrypt-region.
(pgg-pending-timers): A new hash for tracking the passphrase cache
timers, so that new ones supercede old ones.
(pgg-add-passphrase-to-cache): Renamed from
`pgg-add-passphrase-cache' to reduce confusion (all callers
changed).  Modified to cancel old timers when new ones are added.
(pgg-remove-passphrase-from-cache): Renamed from
`pgg-remove-passphrase-cache' to reduce confusion (all callers
changed).  Modified to cancel old timers when their keys are
removed from the cache.
(pgg-cancel-timer): In Emacs, an alias for cancel-timer; in
XEmacs, an indirection to delete-itimer.
(pgg-read-passphrase-from-cache, pgg-read-passphrase): Extracted
pgg-read-passphrase-from-cache from pgg-read-passphrase so users
can only check cache without risk of prompting.  Corrected bug in
notruncate  behavior.
(pgg-read-passphrase-from-cache, pgg-read-passphrase)
(pgg-add-passphrase-cache, pgg-remove-passphrase-cache): Added
informative docstrings.
(pgg-decrypt): Convey provided passphrase in subordinate call to
pgg-decrypt-region.
(pgg-encrypt-region, pgg-encrypt-symmetric-region)
(pgg-encrypt-symmetric, pgg-encrypt, pgg-decrypt-region)
(pgg-decrypt, pgg-sign-region, pgg-sign): Add optional
'passphrase' argument, so the passphrase can be managed externally
and then passed in to the system.
(pgg-read-passphrase, pgg-add-passphrase-cache)
(pgg-remove-passphrase-cache): Add optional 'notruncate' argument,
so the passphrase cache can be used reliably with identifiers
besides a pgp packet's key id.
(pgg-encrypt-symmetric, pgg-encrypt-symmetric-region):
New user commands for symmetric encryption.