Enable retrieving key from URI.
+ Enable the curl code that was already there.
+ Fixes #zfsrogue/zfs-crypto/issues/24.
If keyformat == raw, we need to duplicate the value
into tmpkeydata and tmpkeydatalen so that the tripple check
at the bottom of key_hdl_to_zc() works.
Fixes https://github.com/zfsrogue/zfs-crypto/issues/23.
Add encryption information to zfs manpage
+ Rip (and modify slightly) zfs_encrypt.1 from OpenSolaris11.
Fixes https://github.com/zfsrogue/zfs-crypto/issues/21.