libthread: run first thread in proc on system stack

For pthread systems that are fussy about which stack is used,
this makes sure that threadmain runs on a system stack.
If you only use proccreate (never threadcreate), all threads run
on system stacks.

man/man3/thread.3

@@ -169,6 +169,14 @@ initialized to the desired value
 .B mainstacksize
 .B =
 .BR 1024 ).
+When using the
+.I pthread
+library, 
+.B mainstacksize
+is ignored, as is the stack size argument to
+.BR proccreate :
+the first thread in each proc
+runs on the native system stack.
 .PP
 .I Threadcreate
 creates a new thread in the calling proc, returning a unique integer