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