Granularity of zcompile header magic

[Bart Schaefer <schaefer@xxxxxxxxxxxxxxxx>]

It's possible that we're encountering spurious errors because changes in
the structure of exec.c and/or parse.c invalidate the wordcode in zcompile
output files, but that's not detected by autoload because the header only
contains $ZSH_VERSION (and not the patchlevel).

Originally this was done with the expectation that most people would need
to re-zcompile when installing a major new release, but maybe we should be
bumping along in smaller steps -- or at least have a mechanism for doing
some bumping when a wordcode-altering change is made?

See parse.c: write_dump() and load_dump_header(), though I can't think of
any simple way to automate such version-bumping (other than to just use
the full ZSH_PATCHLEVEL).