MFC: r253266

Prefix the alias macros for members of struct __mcontext with an underscore
in order to avoid a clash in the net80211 code.
MFC: r253742

- Add const-qualifiers to the arguments of isonum_*().
- According to ISO 9660 7.1.2, isonum_712() should return a signed value.
- Try to get isonum_*() closer to style(9).