RFE: add support for "else if" operator

[recck]

rpmbuild currently allows us to:

%if something
...
%elif something-else
...
%else
...
%endif

For parity between each, debbuild should add an %elif operator or similar. Currently to mimic an elseif in debbuild you need to something like:

%if something
...
%else
%if something-else
...
%else
...
%endif # to close something-else
%endif # to close else from "if something"

Related issue from RPM: rpm-software-management/rpm#311

[Conan-Kudo]

Related note: RPM introduced %elif, %elifos, and %elifarch to complement the %if, %ifos, and %ifarch conditionals.

[ascherer]

See this commit for rpm for technical details. (On Kubuntu 20.04, I only have rpmbuild 4.14.2.1 amd can't test this feature, which appeared in rpm 4.15.)