Preparing to move logger calls out of lower-level libaries, and
propagating error details to the caller instead.

Regression by commit bbdf2dcf

Since commit cc64c715, UpdateService::Enqueue() throws
ProtocolError(ACK_ERROR_UPDATE_ALREADY) instead of returning 0.

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.

Modeled after std::filesystem::operator/() from C++17.

Supports .mpdignore on NFS/SMB and others (closes #290).