os: add support for %f to pnprintf

This is the lazy man's %f support. Print the decimal part of the number,
then append a decimal point, then print the first two digits of the
fractional part. So %f in sigsafe printing is really %.2f.

No boundary checks in place here.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Keith Packard <keithp@keithp.com>

include/misc.h

@@ -250,6 +250,7 @@ extern char **xstrtokenize(const char *str, const char *separators);
 extern void FormatInt64(int64_t num, char *string);
 extern void FormatUInt64(uint64_t num, char *string);
 extern void FormatUInt64Hex(uint64_t num, char *string);
+extern void FormatDouble(double dbl, char *string);
 
 /**
  * Compare the two version numbers comprising of major.minor.