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.
Correct typo in vmod_std example
Import latest version from github.com/varnish/varnish-devicedetect/
Use exit() rather than _exit() so we can dump gcov info.
Update README to point out how to contribute patches and bug reports.

Thanks to James Pearson for the suggestion.
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.
Bump version numbers in preparation for 3.0.5