remove verify_src_file

remove verify_src_file

Splitting out this function bothered me. It is conceptially similar to
file_pruned, and yet also very specific to exactly the security needs of
find_src_files.

I liked that it got rid of duplicate code in the latter function. So
instead, put a helper sub in that, which I think allows refactoring
things more cleanly, and with less boilerplate.

As to the needs of gen_autofile, I'm not convinced this needs to handle
the same set of problems that verify_src_file did. So I sat down and
wrote a custom validator for autofiles, which turned out to seem to just
need three things: Make sure the candidate filename is not something
that would be pruned; untaint the candidate filename; and make sure that
srcdir doesn't already have something with its name. (Plus, of course,
all the other checks that were already in gen_autofile.)

(In passing, also fixed a bunch of bugs I had introduced in this branch.)