[libj6] Add log area and severity to j6::syslog()

User code can now set the log area and severity of log messages. This also updates the j6_log
syscall to take these parameters, but removes all calls to it except through j6::syslog().

definitions/syscalls.def

@@ -24,6 +24,8 @@ interface syscalls [syscall] {
 
     # Write a message to the kernel log
     function log {
+        param area uint8
+        param severity uint8
         param message string
     }