bug#14894: 24.3.50; :enable or :visible for encryption menu items

[Drew Adams <drew.adams@oracle.com>]

I know nothing about file encryption, so feel free to close this if it
is not relevant.

I'm guessing that it makes no sense to try to decrypt a file that is not
encrypted, and that it makes no sense to encrypt a file that is already
encrypted.  Perhaps similarly for verifying or signing.

If that is so, then the menu items should have :enable or :visible to
reflect these conditions.

Beyond the menu items, the associated non-Dired commands/functions
should perhaps raise an error or at least show a message if the
requested operation cannot be performed for the given file.

Again, if this does not make sense, please close the bug.  Thx.

In GNU Emacs 24.3.50.1 (i686-pc-mingw32)
 of 2013-07-01 on LEG570
Bzr revision: 113246 lekktu@gmail.com-20130701165437-ea20s94hqwp3ttaj
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
 `configure --prefix=/c/usr --enable-checking CFLAGS='-O0 -g3'
 CPPFLAGS='-DGLYPH_DEBUG=1 -I/c/usr/include''