In mysqld, all local messages ("things found on
console / in error log") now go through a central
function rather than through a lot of fprint()s.
This means we can properly timestamp those lines
with ISO 8601 compliant timestamps (likewise for
slow and general log files); it also means that
we can gag those messages with the new system
variable / startup option --log_error_verbosity,
which depraces --log_warnings / -W.
As timestamps go, the DBA can choose between system
time or zulu time with the new option --log_timestamps.

Client programs now print their warnings prefixed by
their name (no path, no ".exe") and the severity level
in brackets, so "Warning: foo" from bin/mysql.exe
becomes "mysql: [Warning] foo"
This is useful especially in mysql_upgrade (which in
turn calls other binaries) so we can see whether a
warning was thrown by mysql_upgrade or one of its
children (and in the latter case, which).

On the tech side, we now have a generic hook for
local messages. This prints to stderr by default
(and in the case of client apps likely always will),
but can be overridden to log elsewhere, mangle the
string, etc. (as is done by the server once the
facilities are initiliazed; then this goes to
error_log_print, for better timestamps, filtering by
verbosity level, and for printing to NT syslog on Win).

We deprecate my_printf_warning() which does something
similar, with a less clear name.

New test cases added, comments clarified, old test cases
updated.