Since we switched from autotools to Meson in commit
94592c14, 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.

Required according to RFC 3986:

> An implementation should accept uppercase letters as equivalent to
> lowercase in scheme names

Closes #330

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.

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

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++.