terminal,service: better printing of suspended breakpoints (#3415)

Show the location expression that will be used to set a suspended
breakpoint in the breakpoints list.

Also change 'target' called without arguments to print a better error
message and 'target follow-exec' without the last argument to print the
state of follow-exec.

pkg/proc/breakpoints.go

@@ -1018,6 +1018,7 @@ type SetBreakpoint struct {
 	File         string
 	Line         int
 	Expr         func(*Target) []uint64
+	ExprString   string
 	PidAddrs     []PidAddr
 }