Use zstd dictionaries.

Since we have small strings, they can benefit from some shared context,
and zstd supports this. plocate-build now reads the mlocate database
twice; the first pass samples 1000 random blocks, which it uses to train
a 1 kB dictionary. (zstd recommends much larger dictionaries, but practical
testing seems to indicate this doesn't help us much, and might actually
be harmful.)

We get ~20% slower builds and ~7% smaller .db files -- but more
interestingly, linear search speed is up ~20% (which indicates that
decompression in itself benefits more). We need to read the 1 kB
dictionary, but it's practically free since it's stored next to the
header and so small.

This is a version bump (to version 1), so we're not forward-compatible,
but we're backward-compatible (plocate still reads version 0 files
just fine). Since we're adding more fields to the header anyway,
we can add a new “max_version” field that allows for marking
backwards-compatible changes in the future, ie., if plocate-build
adds more information that plocate would like to use but that older
plocate versions can simply ignore.