This is just in preparation for exposing the handle in some APIs.
No functional changes.

Just preparing to export in the next steps. No functional changes.

Add rpmPushMacroAux() function which allows registering auxiliary macros
implemented in C and some required helper facilities: export the types,
rpmMacroBufAppend*() and rpmMacroBufErr() functions, and finally
add rpmMacroEntryPriv() to let macro retrieve their private data.
And that's all folks.

This stupid syntax is the thing preventing %prep from becoming a normal
script, and that in turn blocks so many things now that this has got to
go.

Considering how widely used this syntax is, this is a disruptive
and abrupt change - we only started warning on %patchN in 4.18. So lets
at least print out a legible error message with suggestions how to fix
for now, even if it requires specifically checking for the obsolete
syntax.

Now that we can, implement %setup and %patch as auxiliary macros that are
defined only during %prep parsing. And voila, %prep is no longer special
at all. Not much anyhow.

Notably rpmspec --parse now emits %build where it belongs.
The error message on %patchN is now very much an ugly hack, but at least
it's something we can drop one sunny day.

Fixes: rpm-software-management#2205