Merge load_playlist() into playlist_open() and simplify.

Merge load_playlist() into playlist_open() and simplify.

They are reasonably small. Remove the weird calling convention with
the PLAYLIST_LOADED error code and the pointless check for the dummy
row because they only obfuscate the code. Moreover, the comment of
load_playlist() is actively misleading because this function is not
a loop callback. Extend the documentation of the combined public
playlist_open() while at it.

The code can be simplified by calling pl_get_def_by_name() rather
than osl_get_row() followed by pl_get_name_and_def_by_row().