S9 EXT  (sys:lock string)  ==>  boolean

Create a lock file named (string-append STRING ".lock"). If
the file already exists when the function is called, return
#F, indicating that the requested resource is already locked.
Otherwise return #T. SYS:LOCK uses mkdir() to create the
lock file, which is atomic on virtually all systems.

(sys:unlock "foo")
(list (sys:lock "foo")
      (sys:lock "foo"))  ==>  (#t #f)
