Make RPM Provide: $varnishabi

We want VMODs to depend on the right ABI, so we need to provide
something they can depend on.  Do so, and hook that up with the RPM
build process.
Fix two issues in our backtrace code:

use "nm -an", not "nm -agn" which supresses local symbols.

Let backtrace_symbols() handle anything past _end.
Unhide the functions we emit, so they appear in a backtrace.