proc,terminal: read command line of new processes (#3346)

Read the command line of the main target process as well as any other
process Delve attaches to in follow exec mode.
The command line can be viewed using the 'target list' command.

In follow exec mode this command line is used to match the follow exec
regex to decide whether or not to attach to a child process.

On macOS or when using rr the list of arguments is not available for
attached processes since there is no way to use the gdb serial protocol
to read it.

Fixes #2242

pkg/proc/target.go

@@ -41,7 +41,8 @@ type Target struct {
 	proc   ProcessInternal
 	recman RecordingManipulationInternal
 
-	pid int
+	pid     int
+	CmdLine string
 
 	// StopReason describes the reason why the target process is stopped.
 	// A process could be stopped for multiple simultaneous reasons, in which
@@ -160,7 +161,7 @@ func DisableAsyncPreemptEnv() []string {
 
 // newTarget returns an initialized Target object.
 // The p argument can optionally implement the RecordingManipulation interface.
-func (grp *TargetGroup) newTarget(p ProcessInternal, pid int, currentThread Thread, path string) (*Target, error) {
+func (grp *TargetGroup) newTarget(p ProcessInternal, pid int, currentThread Thread, path, cmdline string) (*Target, error) {
 	entryPoint, err := p.EntryPoint()
 	if err != nil {
 		return nil, err
@@ -182,6 +183,7 @@ func (grp *TargetGroup) newTarget(p ProcessInternal, pid int, currentThread Thre
 		fncallForG:    make(map[int64]*callInjection),
 		currentThread: currentThread,
 		pid:           pid,
+		CmdLine:       cmdline,
 	}
 
 	if recman, ok := p.(RecordingManipulationInternal); ok {