KCall methods do not depend on m_source and m_type fields

- substituted the use of the m_source message field by
  caller->p_endpoint in kernel calls. It is the same information, just
  passed more intuitively.
  
- the last dependency on m_type field is removed.
  
- do_unused() is substituted by a check for NULL.

- this pretty much removes the depency of kernel calls on the general
  message format. In the future this may be used to pass the kcall
  arguments in a different structure or registers (x86-64, ARM?) The
  kcall number may be passed in a register already.

kernel/system/Makefile.inc

@@ -3,7 +3,6 @@
 
 .PATH:	${.CURDIR}/system
 SRCS+= 	\
-	do_unused.c \
 	do_fork.c \
 	do_exec.c \
 	do_newmap.c \