Clean up "allow to"

* README, cipher/cipher.c, cipher/pubkey.c, doc/gpg.texi: replace
  "allow to" with clearer text

In standard English, the normal construction is "${XXX} allows ${YYY}
to" -- that is, the subject (${XXX}) of the sentence is allowing the
object (${YYY}) to do something. When the object is missing, the
phrasing sounds awkward, even if the object is implied by context.
There's almost always a better construction that isn't as awkward.

These changes should make the language a bit clearer.

• Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>