Add an extra token to each line of stderr from termshark processes

For example:

time="2022-07-02T12:46:22-04:00" level=error msg="tshark: Configuration Profile \"test1\" does not exist" cmd=/home/gcla/bin/tshark

This might make it a little easier to understand when a tshark process
fails to run correctly.

I may go further and capture stderr for display in a UI error dialog.

pcap/cmds.go

@@ -46,7 +46,7 @@ func (c *Command) String() string {
 func (c *Command) Start() error {
 	c.Lock()
 	defer c.Unlock()
-	c.Cmd.Stderr = log.StandardLogger().Writer()
+	c.Cmd.Stderr = termshark.ErrLogger("cmd", c.Path)
 	c.PutInNewGroupOnUnix()
 	res := c.Cmd.Start()
 	return res

utils.go

@@ -674,6 +674,13 @@ func SaveOffsetToConfig(name string, offsets2 []resizable.Offset) {
 
 //======================================================================
 
+func ErrLogger(key string, val string) *io.PipeWriter {
+	l := log.StandardLogger()
+	return log.NewEntry(l).WithField(key, val).WriterLevel(log.ErrorLevel)
+}
+
+//======================================================================
+
 // Need to publish fields for template use
 type JumpPos struct {
 	Summary string `json:"summary"`