Fixed segfault, !userlog command syntax and column check

Final !userlog syntax:
!userlog [<lines>] [<column> <search pattern>]

Columns: nick, cid, addr, credentials, useragent, message, time, all

Valid columns are printed if invalid is entered, column must be always specified if searching for a given pattern. One special column "all" is added for unspecified column.
Warning using column "all" can result in very long search time and hub lag!

Examples:
!userlog ... prints last 20 lines
!userlog 10 ... prints last 10 lines
!userlog 5 nick tester ... searches for "tester" in column "nick", prints 5 last results