implement for the "load" command the same logic used for the "add"
command: local clients can load playlist specified as absolute paths.
For relative paths the old logic is preserved: first look for a stored
playlist, then look in the music directory.
Since we switched from autotools to Meson in commit
94592c1406, 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.
This adds a bit of overhead, but also adds flexibility to the API,
because arbitrary triggers may be invoked from that virtual method
implementation, not just Cond::signal().
The motivation for this is to make the handlers more dynamic, for the
upcoming buffering class utilizing ProxyInputStream.
