Give the mlinks and keys tables a pageid index,

as suggested by jeremy@ and espie@.

The mlinks index speeds up basic apropos(1) searches by around 30%
because it speeds up the final SELECT FROM mlinks query by about 95%.
For large result sets, the overall speedup gets even larger, in the
extreme case of "apropos Nd~." by more than 90%.
The keys index finally makes the apropos(1) -O option usable: It no longer
incurs relevant extra cost, while in the past it was embarrassingly slow.

This comes at a cost:  Total database build times grow by about 5%,
and each index adds about 10% database size with -Q.  I consider that
acceptable in view of the huge apropos(1) performance gains.
The -Q database for /usr/share/man still remains below 1 MB.

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