libzdb: Initial breakout of libzdb

Step 1 in trying to slowly rip the zdb functions out of zdb.c
to allow people to play with more flexible things to leverage
zdb's functionality.

No promises on any functions or structs being stable, now or probably
in general unless someone builds a more polished abstraction, the
goal at the moment is to slowly untangle the global state usage
in zdb...

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rich Ercolani <rincebrain@gmail.com>
Closes #15804