Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

1) Const enough things to avoid warnings.
2) Cast ifdef_level to a size_t before comparing it to a ratio of size_ts.
Ifdef_level should always be positive.
3) Complete prototype for chfont.
4) Cast some ptrdiff_ts to ints before using as a field width.
5) Avoid shadowing a local variable p with another local variable p.

Make this compile cleanly when warnings are enabled:
- ANSIfy function declarations
- braces around initializers structs within structs
- add parens in complicated expressions
- disambiguate dangling elses
- no more implicit int
- make functions static where possible
- use prototypes
- don't use varargs hack for diag()

Requested by: joerg
MFC after: 2 weeks