preserve-libs.eclass - preserve libraries after SONAME changes
preserve_old_lib <libs to preserve> [more libs]
These functions are useful when a lib in your package changes ABI SONAME.
An example might be from libogg.so.0 to libogg.so.1. Removing libogg.so.0
would break packages that link against it. Most people get around this
by using the portage SLOT mechanism, but that is not always a relevant
solution, so instead you can call this from pkg_preinst. See also the
preserve_old_lib_notify <libs to notify> [more libs]
Spit helpful messages about the libraries preserved by preserve_old_lib.