Since we switched from autotools to Meson in commit
94592c14, we don't need to include
`config.h` early to properly enable large file support.  Meson passes
the required macros on the compiler command line instead of defining
them in `config.h`.

This means we can include `config.h` at any time, whenever we want to
check its macros, and there are no ordering constraints.

Only _WIN32 is defined by the compiler, and WIN32 is not standardized
and may be missing.

Closes #169

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

Fixes false negatives:

 http://foo/dav/example.ogg mismatches http://foo/dav/

.. because StringAfterPrefix() returns just "example.ogg", without
trailing slash (it existed, but was eaten already by the base
matcher).

New in clang 3.6.

Move code from Path::RelativeFS() and make it generic.

After we've been hit by Large File Support problems several times in
the past week (which only occur on 32 bit platforms, which I don't
have), this is yet another attempt to fix the issue.

The soure file client.c has nearly 1000 lines, time for splitting it
into smaller pieces to improve readability.

Prepare splitting client.c into several sources.

Make the client list management a separate sub-library.

The client code uses portable GLib I/O functions and doesn't need the
OS specific socket headers.

Several functions work with the wrong return type, this patch fixes
them.