Log message:
Fix a bug in "?label" expression evaluation, which caused those expressions
to result in an "non-existent label" error when testing for the existence
of a label that does not yet exist (which is the whole point of that
expression).

Bump PKGREVISION to 1.

Log message:
Add a package for z80asm, a Z80 assembler.

This is distinct from the other z80-asm package.  Why do we have it, then?
Because this Z80 assebler has some features that the z80-asm assembler
does not.