Avoid strlen calls after sprintf

Avoid strlen calls after sprintf

* src/file.c (file_timestamp_sprintf):
* src/function.c (func_words, func_call):
* src/job.c (child_error):
* src/main.c (define_makeflags):
* src/output.c (message, error, fatal):
Use return value from sprintf instead of calling strlen
on the resulting buffer.