Generate ciphers with a better entropy.

Unless highRandomQuality=false (or --fast) is set, use Libgcypt's
'GCRY_VERY_STRONG_RANDOM' level by default for cipher generation, like
it's done for OpenPGP key generation.

On the assistant side, the random quality is left to the old (lower)
level, in order not to scare the user with an enless page load due to
the blocking PRNG waiting for IO actions.

Test.hs

@@ -735,6 +735,7 @@ test_crypto = "git-annex crypto" ~: intmpclonerepo $ when Build.SysConfig.gpg $
 			, "type=directory"
 			, "encryption=" ++ Utility.Gpg.testKeyId
 			, "directory=dir"
+			, "highRandomQuality=false"
 			]
 		initremote @? "initremote failed"
 		initremote @? "initremote failed when run twice in a row"