Replace the Berkeley-DB based mandocdb(8) by an SQLite3-based version,

Replace the Berkeley-DB based mandocdb(8) by an SQLite3-based version,
aiming for more flexible development and optimization options.
Kristaps started this during the summer 2012, i did some very heavy
bugfixing during t2k13 and finally, during the last few days,
got it to a state where it is ripe for in-tree development.
Beware, neither the user interfaces nor the database formats
are expected to be stable just yet.

Will not be installed or activated until further discussion.
No functional change to mandoc(1).

"As long as it remains off until we decide the cost, fine." deraadt@