sys/linux: Unify IPC_Flags and IPC_Mode bitsets

In #5399 it was noticed, that IPC_Mode isn't being used within SystemV
IPC procedures, even though it was designed this way, which lead to a
weird API where in order to call SystemV IPC procedures multiple
transmutes and a bitwise-OR are needed.

This unifies IPC_Mode and IPC_Flags bitsets, making it possible to call
the SystemV IPC procedures without extra casts, and rearranges the flags
in a way that hopefully makes it easier to see when they are not
colliding and should not be mixed. The explanation, explaining this
arrangement of the enum was added.

The IPC_Perm structure is modified, so that the flags can be re-used
between the calls. It's probably not as good as keeping them separate,
but should work... hopefully.

Kept the "old" style of documentation for consistency.

Signed-off-by: Sunagatov Denis <thebumboni@gmail.com>

core/sys/linux/bits.odin

@@ -1618,36 +1618,39 @@ PER_HPUX        :: 0x0010
 PER_MASK        :: 0x00ff
 
 /*
-	Bits for access modes for shared memory
+	Bits for SystemV IPC flags.
+
+	In this enum, access modes are common for any shared memory. Prefixed
+	entries (i.e. `IPC_` or `SHM_`) denote flags, where `IPC_` are common flags
+	for all SystemV IPC primitives, and `SHM_`, `SEM_` and `MSG_` are specific
+	to shared memory segments, semaphores and message queues respectively.
+	
+	These bits overlap, because they are meant to be used within the
+	context of specific procedures. Creation flags, used for `*get` procedures,
+	and usage flags used by all other IPC procedures. Do not mix creation and
+	usage flags, as well as flags prefixed differently (excluding `IPC_`
+	prefix).
 */
-IPC_Mode_Bits :: enum {
+IPC_Flags_Bits :: enum {
+	// Access modes for shared memory.
 	WROTH  = 1,
 	RDOTH  = 2,
 	WRGRP  = 4,
 	RDGRP  = 5,
 	WRUSR  = 7,
 	RDUSR  = 8,
-	DEST   = 9,
+	// Creation flags for shared memory.
-	LOCKED = 10,
-}
-
-/*
-	Shared memory flags bits
-*/
-IPC_Flags_Bits :: enum {
 	IPC_CREAT     = 9,
 	IPC_EXCL      = 10,
-	IPC_NOWAIT    = 11,
-	// Semaphore
-	SEM_UNDO      = 9,
-	// Shared memory
 	SHM_HUGETLB   = 11,
 	SHM_NORESERVE = 12,
+	// Usage flags for shared memory.
+	IPC_NOWAIT    = 11,
+	SEM_UNDO      = 9,
 	SHM_RDONLY    = 12,
 	SHM_RND       = 13,
 	SHM_REMAP     = 14,
 	SHM_EXEC      = 15,
-	// Message queue
 	MSG_NOERROR   = 12,
 	MSG_EXCEPT    = 13,
 	MSG_COPY      = 14,

core/sys/linux/types.odin

@@ -937,17 +937,12 @@ IO_Vec :: struct {
 }
 
 /*
-	Access mode for shared memory
+	Access modes and flags used by SystemV IPC procedures.
-*/
-IPC_Mode :: bit_set[IPC_Mode_Bits; u32]
-
-/*
-	Flags used by IPC objects
 */
 IPC_Flags :: bit_set[IPC_Flags_Bits; i16]
 
 /*
-	Permissions for IPC objects
+	Permissions for SystemV IPC primitives.
 */
 IPC_Perm :: struct {
 	key:  Key,
@@ -955,7 +950,7 @@ IPC_Perm :: struct {
 	gid:  u32,
 	cuid: u32,
 	cgid: u32,
-	mode: IPC_Mode,
+	mode: IPC_Flags, // Only contains mode flags.
 	seq:  u16,
 	_:    [2 + 2*size_of(int)]u8,
 }