IPC privileges fixes

Kernel:
o Remove s_ipc_sendrec, instead using s_ipc_to for all send primitives
o Centralize s_ipc_to bit manipulation,
  - disallowing assignment of bits pointing to unused priv structs;
  - preventing send-to-self by not setting bit for own priv struct;
  - preserving send mask matrix symmetry in all cases
o Add IPC send mask checks to SENDA, which were missing entirely somehow
o Slightly improve IPC stats accounting for SENDA
o Remove SYSTEM from user processes' send mask
o Half-fix the dependency between boot image order and process numbers,
  - correcting the table order of the boot processes;
  - documenting the order requirement needed for proper send masks;
  - warning at boot time if the order is violated

RS:
o Add support in /etc/drivers.conf for servers that talk to user processes,
  - disallowing IPC to user processes if no "ipc" field is present
  - adding a special "USER" label to explicitly allow IPC to user processes
o Always apply IPC masks when specified; remove -i flag from service(8)
o Use kernel send mask symmetry to delay adding IPC permissions for labels
  that do not exist yet, adding them to that label's process upon creation
o Add VM to ipc permissions list for rtl8139 and fxp in drivers.conf

Left to future fixes:
o Removal of the table order vs process numbers dependency altogether,
  possibly using per-process send list structures as used for SYSTEM calls
o Proper assignment of send masks to boot processes;
  some of the assigned (~0) masks are much wider than necessary
o Proper assignment of IPC send masks for many more servers in drivers.conf
o Removal of the debugging warning about the now legitimate case where RS's
  add_forward_ipc cannot find the IPC destination's label yet

kernel/priv.h

@@ -36,9 +36,7 @@ struct priv {
 				 */
 
   short s_trap_mask;		/* allowed system call traps */
-  sys_map_t s_ipc_from;		/* allowed callers to receive from */
   sys_map_t s_ipc_to;		/* allowed destination processes */
-  sys_map_t s_ipc_sendrec;	/* allowed sendrec processes */
 
   /* allowed kernel calls */
 #define CALL_MASK_SIZE BITMAP_CHUNKS(NR_SYS_CALLS)
@@ -78,6 +76,8 @@ struct priv {
 #define id_to_nr(id)	priv_addr(id)->s_proc_nr
 #define nr_to_id(nr)    priv(proc_addr(nr))->s_id
 
+#define may_send_to(rp, nr) (get_sys_bit(priv(rp)->s_ipc_to, nr_to_id(nr)))
+
 /* The system structures table and pointers to individual table slots. The 
  * pointers allow faster access because now a process entry can be found by 
  * indexing the psys_addr array, while accessing an element i requires a