Eliminate input_stream_init() and input_stream_deinit().

This is the version present in Ubuntu Oneiric, the oldest distribution
with gcc 4.6.  Debian Squeeze is off target, because it has gcc 4.4,
which is unable to compile MPD anyway.

This commit drops all API compatibility hacks for older versions.

The underlying library has been obsolete for many years.

Works around gcc 4.6 bug.

Usually, when strndup() is not available, strndup() isn't either,
because both are POSIX 2008.

The libwrap version on Mac OS X is broken, it will not work with C++
due to broken function prototypes.

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