bin/12316 more completely.
Highlights:
* we only allow explicitly specified set of (numberic) formats
* %% is correctly skipped, extra %foo formats are detected
* '*' in format specification is not allowed
The code in getformat() is slighly different to the OpenBSD version
of fix - the list of allowed printf(3) flags is explicit and the
condition checking 'chardata' is written slighly differently.
Also pull-in some cosmetic changes from OpenBSD - s/isdefault()/is_default()/,
error() -> errx(), usage().
