An error here will typically mean that the directory was removed between
the time we iterated the parent and the time we wanted to visit it in
which case we should ignore it.

Other kinds of errors such as permissions (or transient errors) also
better dealt with by pretending we didn't see it.

Fix macro redefinition warning

List a submodule only once when the path matches a submodule in the index

The header src/cc-compat.h defines portable format specifiers PRIuZ, PRIdZ, and PRIxZ. The original report highlighted the need to use these specifiers in examples/network/fetch.c.  For this commit, I checked all C source and header files not in deps/ and transitioned to the appropriate format specifier where appropriate.

If we get the path from the gitmodules file, look up the submodule we're
interested in by path, rather then by name. Otherwise we might get
duplicate results.

Clean up some warnings

Fix undefined reference with old versions of openssl