vsnprintf: Expand \n to \r\n for console output

vsnprintf: Expand \n to \r\n for console output

xenconsoled doesn't automatically convert \n into \r\n, which causes test
output to appear like this in some terminals:

  [root@host ~]# xl create -c tests/selftest/test-pv64-selftest.cfg
  Parsing config from tests/selftest/test-pv64-selftest.cfg
  --- Xen Test Framework ---
                            Environment: PV 64bit (Long mode 4 levels)
                                                                      XTF Selftests

There are a number of ways to do this, but by far the most efficient way is to
have vsnprintf() expand \n's in the output buffer.

This however is non-standard behaviour for vsnprintf().  Rename it to
vsnprintf_internal() and take extra flags, and have vprintk() use the new
LF_TO_CRLF control flag.

Inside vsnprintf_internal(), rearrange the non-format and %c logic to share
the expansion logic, as well as extending the logic to fmt_string().

Extend the selftests to confirm correct behaviour in both modes, for all ways
of being able to pass newline characters into a format operation.

Reported-by: Pawel Wieczorkiewicz <wipawel@amazon.de>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>