This way, IsFileNotFound() can detect status 404.

Allows callers to extract the NFS error code.

See commit be137a19

For remote files (not streams), this downloads as quickly as possible
to a large buffer instead of throttling the stream during playback.
Throttling can make the server impatient and it may then disconnect.

This is what Qobuz and Tidal do, and this commit attempts to solve
this by not letting the Qobuz/Tidal server wait (closes #241).

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.

This is not a plugin.

Those are loaded with the "curl" input plugin, and this one is
"expensive", because it needs to send a new HTTP request with Range
header for each seek.

Fixes crash due to "pure virtual method called" in the "mms" input
plugin.  Closes #253

Without setting the "ready" flag, the caller will wait in WaitReady()
forever, locking up MPD.  Closes #252

Requires Android API 21, but we want to support older versions as
well.

To be used when a URL gets added.

This change was missing in 47fa8c4c

The CurlRequest constructor can throw exceptions, therefore we must
not use "noexcept".

Allows catchers to inspect the HTTP status.