fake_outputs: Use %n format specifier instead of sprintf

fake_outputs: Use %n format specifier instead of sprintf

fake_outputs_init used a sprintf invocation with a throw-away buffer
to estimate how many characters the sscanf invocation consumed. This
was unnecessary, and also potentially incorrect, as differences
between the read and formatted strings (such as leading zeros) could
lead to fake_outputs_init to lose its track.

Instead, use the %n format specifier which allows saving the number of
characters consumed by sscanf so far. %n is part of C99.