Support for counting screen sessions running with extra parameters

In some cases (multi user enabled in screen for example), the count of
detached screen sessions was incorrect. The command to locate these has
been improved using a more tolerant regex, which does not require
'detached' to be the first word inside the parenthesis.

To test, open a new screen session, add multiuser (type Ctrl+A, then
':', then multiuser on), then detach (Ctrl+A, Ctrl+D), and see that
liquidprompt detects the screen, as would "screen -list".