'\0' not properly escaped in some manual pages
While helping test a new mandoc update, I discovered that there were several differences. While most of these were expected some of them highlighted a problem that we had two manual pages that used the
'\0' construct without escaping. This is valid syntax per
mandoc_char that doesn't translate to what we think it does. The fix is to escape it properly for ctfdump(1) and for the mbrtoc16(3C) page to probably just switch to a
memset of a literal 0 in its example.