Cleanup zed logging

This is a set of minor cleanup changes related to zed logging:
- Remove the program identity prefix from messages written to stderr
  since systemd already prepends this output with the program name.
- Replace the copy of the program identity string with a ptr reference.
- Replace "pid" with "PID" for consistency in comments & strings.
- Rename the zed_log.c struct _ctx component "level" to "priority".
- Add the LOG_PID option for messages written to syslog.

Signed-off-by: Chris Dunlap <cdunlap@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #2252

cmd/zed/zed_log.h

@@ -41,7 +41,7 @@ void zed_log_pipe_close_writes(void);
 
 void zed_log_pipe_wait(void);
 
-void zed_log_stderr_open(int level);
+void zed_log_stderr_open(int priority);
 
 void zed_log_stderr_close(void);