WL#7793: Add native support for syslog on unixoid platforms

mysqld did not have native support for traditional unix-style
syslog; it instead used a shell-script wrapper that would pipe
the daemon's output to the syslog.  This implied certain limitations:
- it was impossible to see the syslog status at runtime
- it was impossible to change the syslog status at runtime
- the shell script could interfere with log rotation
- the setup would often result in log lines containing two timestamps

a) add native syslog support for unixoid OSs.

   The following can be set a start-up as well as viewed
   and set at runtime via system variables (requiring the
   SUPER privilege).

   --log_syslog[=0|1]
     on/off switch, defaults to off

   --log_syslog_include_pid[=0|1]
     Include mysqld's process ID on each syslog line?
     Defaults to on.  (UNIX only)

   --log_syslog_facility=local5
     choose the syslog "facility". Well-known facilities
     may be selected by name.
     Defaults to "daemon".  (UNIX only)

   --log_syslog-tag=<string>
     If empty and by default, an ident of "mysqld" is used.
     If e.g. tag "abcd" is supplied, "mysqld-abcd" will be used.
     (The hyphen will be used automatically, the user need not
     supply it.)

b) refactoring: mysys/my_syslog.cc and sql/log.cc had
   some duplication of effort with regard to the
   Windows EventLog.  This patch attempts to eliminate
   the duplicates from log.cc, and merge the remaining
   useful code into my_syslog.cc so that other binaries
   may have a chance to benefit from it.

c) Support features of a) on Windows where possible.

d) Update mysqld_safe to use features of a) instead of
   trying to emulate them in the wrapper script where
   possible, with as little change in syntax as possible.