Fixes -Winconsistent-missing-override (clang 3.6).

Replace InputPlugin attributes.

Replaces the method Close().

Prepare for adding virtual methods.

Replaces GLib's g_str_has_prefix().

This was a memory leak, because "mime" was a std::string which created
another copy and discarded the allocated buffer.

Prepare to migrate away from GLib.  Currently, we're still using GLib
as a backend.

Replaces GLib's GError.

Omitting it has the same effect as returning false unconditionally.

Eliminate input_stream_init() and input_stream_deinit().

Hide the definition from C code, to prepare the transition to C++.

Add GMutex, GCond attributes which will be used by callers to
conditionally wait on the stream.

Remove the (now-useless) plugin method buffer(), wait on GCond
instead.  Lock the input_stream before each method call.  Do the same
with the playlist plugins.

input_stream_buffer() has a check for "buffer==NULL", so we don't need
an explicit empty implementation.

All close() implementations must call this method.

Major API redesign: don't let the caller allocate the input_stream
object.  Let each input plugin allocate its own (derived/extended)
input_stream pointer.  The "data" attribute can now be removed, and
all input plugins simply cast the input_stream pointer to their own
structure (with an "struct input_stream base" as the first attribute).

After we've been hit by Large File Support problems several times in
the past week (which only occur on 32 bit platforms, which I don't
have), this is yet another attempt to fix the issue.

The "off_t" type may change when you enable or disable large file
support on 32 bit platforms.  This caused severe ABI problems within
MPD when we enabled LFS for the first time: two sources included
config.h and sys/types.h in different order, and had different off_t
sizes - leading to memory corruption because of ABI incompatibility.
This patch attempts to get rid of all public "off_t" uses: it removes
"off_t" from the input_stream ABI/API, and switches to GLib's 64 bit
"goffset" type.  This may hurt 32 bit embedded platforms a tiny bit,
but that's not even measurable.