Draft: abuild: rename man pages for perl vendor modules

There are more than 20 packages in the aports that manually handle conflicting man pages between perl-doc and perl--doc. This tries to solve the issue with a different approach:

Because the code of perl modules is installed into vendor_perl whereas main/perl installs into core_perl, name clashes in the implementation do not result in file conflicts and the implementation can be installed side by side. This renames the man pages with the 3pm suffix for vendor modules to 3pm-vendor, to also allow the man pages to be installed side by side.

Note that this approach is preferred over just removing the "duplicaiton", as in fact two different implementations with the same name are available that may differ in their API and semantics. Hence, separate documentation is also sensible. The postfix 3vendor-pm for vendor modules instead of 3pm (still used for core modules) was chosen because it is alphabetically greater than 3pm, so that perl core man pages are preferred over vendor man pages, if both are available for the same name. Calling man 3vendor-pm foo would still allow to show the vendor man page explicitly.