detect and handle multiple recording errors

[joshi4]

• cmd/record: detect and fix concurrent recording sessions
• server: detect permanent net.OpErr and exit listen loop
• cmd/record: simplify code
• server: removing the socket doesn't end the listener; so we send a shutdown signal
• server: resolve concurrent errors within the server pkg
• cmd/history: check for multiple recordings before user selects commands
• history: shutdown signal should kill the shell