mem/virtual: specify max protection on mmap call in NetBSD and FreeBSD

core/mem/virtual/virtual_openbsd.odin

@@ -0,0 +1,20 @@
+package mem_virtual
+
+import "core:sys/posix"
+
+_reserve :: proc "contextless" (size: uint) -> (data: []byte, err: Allocator_Error) {
+	result := posix.mmap(nil, size, {}, {.ANONYMOUS, .PRIVATE})
+	if result == posix.MAP_FAILED {
+		assert_contextless(posix.errno() == .ENOMEM)
+		return nil, .Out_Of_Memory
+	}
+
+	return ([^]byte)(uintptr(result))[:size], nil
+}
+
+_decommit :: proc "contextless" (data: rawptr, size: uint) {
+	MADV_FREE :: 6
+
+	posix.mprotect(data, size, {})
+	posix.posix_madvise(data, size, transmute(posix.MAdvice)i32(MADV_FREE))
+}