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.

Move code from config_parse_path().

The old struct config_param remains only for top-level string options.

Lifts the artificial 31 bit limit in i386 where
sizeof(long)==sizeof(int).

Now config_param::GetBlockValue() can be used to get signed integers
from the configuration.

The new class Path only holds a string pointer without being
responsible for allocation/deallocation.  The FileSystem.hxx library
accepts Path arguments instead of AllocatedPath, to avoid forcing
callers to allocate another string object.

Return Path::Null() instead of dying from assertion failure.

Migrate the remaining callers to FatalError().

Replaces GLib's GError.

Migrate all callers to use Path directly, instead of doing the
conversion in each caller.