Add exit function in core module.

This function allow to interrupt the script execution. The parameter exit code is returned by pugs when leaving.

doc/userdoc.org

@@ -2540,11 +2540,16 @@ to files for instance) as we will see below.
 
 **** ~core~ provided functions
 
+***** ~exit: Z -> void~
+
+This function interrupts the execution of the script. The integer (~Z~)
+value is the code that is returned when ~pugs~ exits.
+
 ***** ~getAvailableModules: void -> string~
 
 This function that is used in the preamble of this section is a
 function that returns a ~string~ that contains the list of modules that
-are available in the current version of pugs.
+are available in the current version of ~pugs~.
 
 ***** ~getModuleInfo: string -> string~