logger: add support for -I PID to log, e.g., $$ from a shell script

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

man/logger.1

@@ -38,6 +38,7 @@
 .Op Fl f Ar FILE
 .Op Fl h Ar HOST
 .Op Fl H Ar HOSTNAME
+.Op Fl I Ar PID
 .Op Fl m Ar MSGID
 .Op Fl p Ar PRIO
 .Op Fl P Ar PORT
@@ -105,6 +106,16 @@ will be used.
 Send the message to the remote system
 .Ar host
 instead of logging it locally.
+.It Fl I Ar PID
+Like
+.Fl i ,
+but uses
+.Ar PID .
+Useful when logging from shell scripts that send multiple messages.
+E.g., the following arguments might be a useful template:
+.Bd -literal -offset indent
+logger -t $(basename $0) -I $$
+.Ed
 .It Fl i
 Log the process id of the logger process with each line
 .Ql ( LOG_PID ) .