Makes plugins' STREAMFILEs not throw an error on non-existing .txtp
(normally written inside .m3u) treating it like a command/url-ish name.
This lets the TXTP parser to try to opening the virtual file with
commands, as 0-size TXTP with a formatted name is valid, like
"file.adx#l 1.0.txtp" ("play file.adx with 1 loop"), to allow quick
per-file config.
Tries to simplify code, and buffer is not emptied unless we really read
more data (useful for edge reads). get_offset is now reliable (though
never used).
It was implemented for Audacious only to open companion NWA files, but
current version has no need for it. In case there is some obscure thing
that needs it, Audacious's vfs.cc should translate or return realnames
transparently, without having to modify other streamfiles.
