Avoid unchecked snprintf() to make future code auditing easier.

* jk/snprintf-truncation:
  fmt_with_err: add a comment that truncation is OK
  shorten_unambiguous_ref: use xsnprintf
  fsmonitor: use internal argv_array of struct child_process
  log_write_email_headers: use strbufs
  http: use strbufs instead of fixed buffers