PDP10 Update

Optimize DMA transfers to/from memory:
o Avoid shifting by zero
o Transfer complete (and aligned) PDP10 words whenever possible
o Precompute constant masks
o Explicitly hoist common subexpressions (for lazy compilers)

Document Unibus/PDP10 memory mapping

STOP if an unaligned word DMA is attempted.
SCP divots and enhanced help prototype

find_unit could return uptr undefined if find_dev returns the device disabled.
This upsets help routines that expect either a valid uptr or NULL.  Initialize the pointers to NULL.
And, have find_dev return NULL in the uptr as well as returning a NULL dptr.

Picky compiler casts.

Prototype for enhanced help.
Merge branch 'master' of http://github.com/tlhackque/simh