Dpkg::Shlibs::SymbolFile: included files don't need to repeat the header line

Dpkg::Shlibs::SymbolFile: included files don't need to repeat the header line

Adjust the parser to remember the current soname when processing an
included file so that it doesn't need to repeat (and duplicate!) a header
line. Update dpkg-gensymbols's manual page accordingly. And add a
non-regression test for this feature.