Show process ID in prompt for independent worker sessions

When the well-known lock is active, include program name and PID in
the debugger prompt so it's clear which worker has control:

  (rdbg@test_foo.rb#12345)

Also applies to IRB console integration:

  irb:rdbg@test_foo.rb#12345(main):001:0>

This makes process switches obvious when control moves between
workers on continue.

Author: st0012

lib/debug/irb_integration.rb

@@ -21,6 +21,9 @@ def activate_irb_integration
       irb = IRB::Irb.new(workspace)
       IRB.conf[:MAIN_CONTEXT] = irb.context
       IRB::Debug.setup(irb)
+      if (pi = SESSION.process_info)
+        irb.context.irb_name = "irb:rdbg@#{pi}"
+      end
       IRB::Context.prepend(IrbPatch)
     end
   end

lib/debug/session.rb

@@ -493,8 +493,8 @@ def wait_command_loop
     def prompt
       if @postmortem
         '(rdbg:postmortem) '
-      elsif @process_group.multi?
-        "(rdbg@#{process_info}) "
+      elsif (pi = process_info)
+        "(rdbg@#{pi}) "
       else
         '(rdbg) '
       end
@@ -2085,7 +2085,7 @@ def intercept_trap_sigint_end
     end
 
     def process_info
-      if @process_group.multi?
+      if @process_group.multi? || @process_group.wk_locked?
         "#{$0}\##{Process.pid}"
       end
     end
@@ -2155,6 +2155,10 @@ def wk_lock
       @wk_lock_file&.flock(File::LOCK_EX)
     end
 
+    def wk_locked?
+      !multi? && @wk_lock_file
+    end
+
     def unlock_wk_lock
       return if multi?
       @wk_lock_file&.flock(File::LOCK_UN)