execve_runproc: Unbreak __MACHINE_STACK_GROWS_UP machines.  Clarify the stack
address allocation code.  Summarize an awful big comment about the _rtld()
"gap".

(The log message in Rev. 1.384 was wrong; the new stack address is passed
not via the 3rd register argument, but via the SP.  The 3rd is for ps_strings.)