if execinfo.h is not present

the buffer with a null byte. Formatters must always set the length
field -- null-terminated return strings may not be assumed.

efficiently

Also refactor strfTIM* to use the %i formatter for the subsecond
value in microseconds (instead of %N for nanoseconds).
Also refactor the varnishevent time formatters to directly use the
string in the log for subseconds, rather than parsing them as doubles,
to avoid inaccuracies due to floating point.

(temporarily #if'fing out everything that uses it)