inspect-library: Add support for consuming deb-symbols(5) files

(This branch includes !20 (merged))

The Steam Runtime is built using dpkg/apt, so we have this information already for a lot of the shared libraries, for example in /var/lib/dpkg/info/zlib1g:amd64.symbols in a SDK container.

For those that we don't, such as libcurl3, we can generate a deb-symbols(5) file by either improving the packaging, or using for example

dpkg-gensymbols -q -v0 -plibcurl3 -e/usr/lib/x86_64-linux-gnu/libcurl.so.3 -I/dev/null -O

(although the output of such commands will require some postprocessing and common sense to filter out private symbols that are not meant to be part of the ABI).

Format reference: deb-symbols(5)