inspect-library: Treat "-" as meaning stdin

To test:

echo "foo@Base" | ./_build/helpers/x86_64-linux-gnu-inspect-library libz.so.1 -

This isn't used for anything yet, but when we give it more integration into the library, we might want to use a pipe to supply the list of symbols. For instance, if we use deb-symbols(5) symbols lists (which describe one or more shared libraries, each with their symbols) to describe the Steam Runtime, then we'll want something like this pseudocode:

while lines remain:
    SONAME = next line until first whitespace

    run helper with pipes on stdin and stdout

    while next line starts with one of ('|', '*', ' '):
        if next line starts with ' ':
            symbol@version = rest of line until first whitespace
            write symbol@version to pipe
        else:
            ignore '* Field: value' or '| alternative dependency'

    close pipe to helper's stdin
    collect results from stdout
    collect exit status
    add SrtLibrary to list of libraries