Fix Print::printf return value on truncation and error

[l5yth]

• vsnprintf returns the would-be length (not bytes written) and a negative int on encoding error.
• Storing that in size_t and returning it gave callers an overcount on truncation and ~SIZE_MAX on error.
• Clamp to what actually fit and return write()'s byte count.
• Fix Print::printf returns an incorrect byte count #72

[CLAassistant]

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