zdb: add decryption support

The approach is straightforward: for dataset ops, if a key was offered,
find the encryption root and the various encryption parameters, derive a
wrapping key if necessary, and then unlock the encryption root. After
that all the regular dataset ops will return unencrypted data, and
that's kinda the whole thing.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Jorgen Lundman <lundman@lundman.net>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Closes #11551
Closes #12707
Closes #14503

cmd/zdb/Makefile.am

@@ -1,4 +1,5 @@
 zdb_CPPFLAGS = $(AM_CPPFLAGS) $(FORCEDEBUG_CPPFLAGS)
+zdb_CFLAGS   = $(AM_CFLAGS) $(LIBCRYPTO_CFLAGS)
 
 sbin_PROGRAMS   += zdb
 CPPCHECKTARGETS += zdb
@@ -12,3 +13,5 @@ zdb_LDADD = \
 	libzpool.la \
 	libzfs_core.la \
 	libnvpair.la
+
+zdb_LDADD += $(LIBCRYPTO_LIBS)