Replace "extern inline" with "static inline"

Replace "extern inline" with "static inline"

A number of headers define functions as "extern inline" which is
causing problems with gcc5.  The reason is that starting with
version 5.1, gcc defaults to the standard C99 semantics for the
inline keyword.

Under the traditional GNU inline semantics, an "extern inline"
function would never create an external definition, the same
as inline *without* extern in C99.  In C99, and "extern inline"
definition is simply an external definition with an inline hint.
In short, the meanings of inline with and without extern are
swapped between GNU and C99.

The upshot is that all these definitions in header files create
an external definition wherever those headers are included,
resulting in multiple definition errors at link time.

Changing all these functions to "static inline" fixes the problem
since this works as desired in all gcc versions.  Although the
semantics are slightly different (a static inline definition may
result in an actual function being emitted), it works as intended
in practice.

This patch also removes extern prototype declarations for the
changed functions where they existed.

Signed-off-by: Mans Rullgard <mans@mansr.com>