Tentatively apply https://reviews.llvm.org/D23921, to get rid of false

Tentatively apply https://reviews.llvm.org/D23921, to get rid of false
positive diagnostics from -Wvarargs about enum parameters, e.g.:

cddl/contrib/opensolaris/lib/libnvpair/libnvpair.c:388:15: error: passing an object that undergoes default argument promotion to 'va_start' has undefined behavior
      [-Werror,-Wvarargs]
        va_start(ap, which);
                     ^
cddl/contrib/opensolaris/lib/libnvpair/libnvpair.c:382:66: note: parameter of type 'enum nvlist_prtctl_fmt' is declared here
nvlist_prtctl_dofmt(nvlist_prtctl_t pctl, enum nvlist_prtctl_fmt which, ...)
                                                                 ^