The API documentation says "move as much data as possible", and if
there is room at the head of the buffer, we should use that if the
room after the tail is not large enough.

The result shall be positive.

Required according to RFC 3986:

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

Closes #330

Since 7d353bbe, _GNU_SOURCE is always
defined, which implies __USE_MINGW_ANSI_STDIO and thus switches to
the mingw implementations of the printf() family.  That's
standards-compliant, unlike Microsoft's CRT implementations.

Since 7d353bbe, _GNU_SOURCE is always
defined, which implies __USE_MINGW_ANSI_STDIO and thus switches to
the mingw implementations of the printf() family.  That's
standards-compliant, unlike Microsoft's CRT implementations.

No longer a problem with NDK r16.

Prepare for option values.