Fix build
pkg_version_cmp() was previously used with the sense reversed to the
replacement pkg_version_change().  Everywhere else it is

   pkg_version_cmp(oldversion, newversion)

and this is as it should be.
To reduce confusion spell out exactly what type of change
pkg_version_change() returns.  This also allows pkg_change_t to be
used directly as an array index in pkg_jobs.c
oldversion can be NULL, which would trigger an assertion in
pkg_version_cmp.