infrastructure for saner ret_from_kernel_thread semantics

* allow kernel_execve() leave the actual return to userland to
caller (selected by CONFIG_GENERIC_KERNEL_EXECVE).  Callers
updated accordingly.
* architecture that does select GENERIC_KERNEL_EXECVE in its
Kconfig should have its ret_from_kernel_thread() do this:
	call schedule_tail
	call the callback left for it by copy_thread(); if it ever
returns, that's because it has just done successful kernel_execve()
	jump to return from syscall
IOW, its only difference from ret_from_fork() is that it does call the
callback.
* such an architecture should also get rid of ret_from_kernel_execve()
and __ARCH_WANT_KERNEL_EXECVE

This is the last part of infrastructure patches in that area - from
that point on work on different architectures can live independently.

Signed-off-by: Al Viro <[email protected]>

arch/Kconfig

@@ -261,6 +261,9 @@ config ARCH_WANT_OLD_COMPAT_IPC
 config GENERIC_KERNEL_THREAD
 	bool
 
+config GENERIC_KERNEL_EXECVE
+	bool
+
 config HAVE_ARCH_SECCOMP_FILTER
 	bool
 	help