available since the interface is poorly defined on at least one major
platform (Solaris).

Moved table of constant names for fpathconf() & pathconf() into the
conditional that defines the conv_path_confname() helper; Mark Hammond
reported that defining the table when none of the constants were
defined causes the compiler to complain (won't allow 0-length array,
imagine that!).

In posix_fpathconf(), use conv_path_confname() as the O& conversion
function, instead of the conv_confname() helper, which has the wrong
signature (posix_pathconf() already used the right thing).