Arrange to update cached LWP credentials in userret() rather than during
syscall/trap entry, eliminating a test+branch on every syscall/trap.

This wasn't possible in the 3.99.x timeframe when l->l_cred came about
because there wasn't a reliable/timely way to force an ONPROC LWP running on
a remote CPU into the kernel (which is just about the only new thing in
this scheme).

usermode/syscall.c: s/fall trough/FALLTHROUGH/ (KNF)

Centralized shared part of child_return() into MI part

Add a new function md_child_return() for MD specific bits only.

New child_return() is now part of MI and central code that handles
uniformly tracing code (KTR and ptrace(2)).

Synchronize value passed to ktrsysret() among ports to SYS_fork. This is
a traditional value and accessing p_lflag to check for PL_PPWAIT shall
use locking against proc_lock. Returning SYS_fork vs SYS_vfork still isn't
correct enough as there are more entry points to forking code. Instead of
making it too good, just settle with plain SYS_fork for all ports.

branches: 1.24.34;
Use sy_invoke

branches: 1.23.6;
Add cpu_spawn_return() as per other archs to make it compile. No tests done
yet.

Reimplement userret() to also include AST for preemption.

Note it would be nice if we could do the check "are we going to userland?" in
a less intrusive way.

Revamp the NetBSD/usermode pagefault and illegal instruction handing. It now
can handle recursive entry and is a lot more memory tight compared to the old
implementation. Performance wise:

* slightly less number of syscalls/sec possible though could be optimized
* a lot faster context creation / destruction making overall operation faster.

Rename the debug printf's to use a thunk_ prefix to avoid confusion.

Use to the MAP_NOSYSCALLS argument to mmap() to allow for NetBSD/usermode to
execute bog-standard native programs.

add required trace_enter/trace_exit calls in syscall() to make ktrace work

Elaborate return value printing to include JUSTRETURN etc. instead of
reporting it as an error!

Enhance system call prints by printing out the execve() arguments

Return ZERO to the child on fork!

Print lwp values on syscall entry

Rework the debug printing to use functions instead of clutting the system call
code.

Enhance printing of system callss

branches: 1.11.2;
Big patch that changes the signal stack usage of urkel significantly.

Formerly, all signals came on the signal stack and the two important ones were
then forwared to either the system call or the pagefault handler. This worked
fine but the signal stack remains that, a stack. When we go multi-process this
stack gets corrupted and out-of-order with all kind of nastyness since a
userland process switch can occure when a system call is called or when a
process gets a page fault.

The new scheme only uses the signal stack as a jumpboard. It swaps states and
then returns from the signal, clearing the stack but instead of returning to
the code it now jumpt to the handler and that handler then returns to the code
when its finished.

branches: 1.10.2;
Move to dprintf_debug() too

machdep.c: Cleanup the machine dependent code and set the carry flag on error
out. While here also clean up register printing.

syscall.c: Add debug syscall printing code to track basic syscalls being
taken.

Implement a working(!) syscall! well, it can call a number of syscalls without
dying ;)

Create syscall() prototype and let illegal instruction handler switch to that
switchframe

Create a ucontext for the system call to work in; its cloned from the new
pcb's call `userland' ucontext.

Update syscall prototype

Clean child_return()

Add dirty copy of child_return()

Drop 3rd and 4th clauses. OK jmcneill (copyright holder).

branches: 1.1.2; 1.1.4; 1.1.6; 1.1.8; 1.1.18;
Import work-in-progress NetBSD/usermode port.