Avoid global declarion of internal functions that are new in 3.17

Avoid global declarion of internal functions that are new in 3.17

_fuse_new() is not supposed to be called by external users
outside of internal functions or static inlined functions.

This also removes several functions from lib/fuse_versionscript
which where added and exported by
commit 58f85bfa9b7d ("Add in the libfuse version a program...)
as these are libfuse internal only.

Signed-off-by: Bernd Schubert <bernd@bsbernd.com>