Fix for 3 minor issues (unrelated to each other)

[davidBar-On]

• Version of iperf3 (or development branch, such as master or
  3.1-STABLE) to which this pull request applies:
  master

• Issues fixed (if any): none

• Brief description of code changes (suitable for use as a commit message):

Fix 3 minor issues that are unrelated to each other, but decided to put them in the same PR to save number of PRs. Issues are:

1. iperf_api.c: allow server to store received data in file.
2. iperf_error.c: add missing case for IETOTALINTERVAL.
3. iperf_locale.c: fix copy/paste error ("bytes" instead of "blocks").