provide a FBUF_STATIC_INITIALIZER_POINTER() macro

to simplify (and speedup) initialization of a static fbuf structure.

This is handy when you have a preallocated structure which contains an fbuf_t member
(not a pointer) and we want to set custom parameters without having to call multiple functions to do that
(and obviously FBUF_STATIC_INITIALIZER can't be used because we are not in a declaration statement)