SYSENTER/SYSCALL support

. add cpufeature detection of both
	. use it for both ipc and kernelcall traps, using a register
	  for call number
	. SYSENTER/SYSCALL does not save any context, therefore userland
	  has to save it
	. to accomodate multiple kernel entry/exit types, the entry
	  type is recorded in the process struct. hitherto all types
	  were interrupt (soft int, exception, hard int); now SYSENTER/SYSCALL
	  is new, with the difference that context is not fully restored
	  from proc struct when running the process again. this can't be
	  done as some information is missing.
	. complication: cases in which the kernel has to fully change
	  process context (i.e. sigreturn). in that case the exit type
	  is changed from SYSENTER/SYSEXIT to soft-int (i.e. iret) and
	  context is fully restored from the proc struct. this does mean
	  the PC and SP must change, as the sysenter/sysexit userland code
	  will otherwise try to restore its own context. this is true in the
	  sigreturn case.
	. override all usage by setting libc_ipc=1

include/minix/const.h

@@ -173,4 +173,8 @@
 /* magic value to put in struct proc entries for sanity checks. */
 #define PMAGIC 0xC0FFEE1
 
+/* MINIX_KERNFLAGS flags */
+#define MKF_I386_INTEL_SYSENTER	(1L << 0) /* SYSENTER available and supported */
+#define MKF_I386_AMD_SYSCALL	(1L << 1) /* SYSCALL available and supported */
+
 #endif /* _MINIX_CONST_H */