Move the last bits of file descriptor awareness out of the "abstract"
director layer.
Allow vcl_init{} to return(fail) in which case vcl.load or vcl.inline
fails too.
Test case for vcl_init -> return(fail)
Use vrt_ctx also for init/fini
Introduce "VRT_CTX" as a shorthand macro for "const struct vrt_ctx *ctx"
Cast y argument to uintmax_t in RDN2/RUP2

If the type of the y argument to RDN2/RUP2 is smaller than the type of
the x argument, these macros would fail to round correctly. Cast to
uintmax_t to avoid this problem.