SDS: Allow LOAD command to accept a more flexible boot sequence

The canonical boot sequence uses WIM 0,2 in location 5 and an initial load address in location 11 (and in the X register). This is limiting for devices like RAD that do not report an end-of-file, or for paper tape that report EOF too late and cause the WIM to fail. A simple load address in location 11, with bit 9 set to satisfy the BRX, doesn't do any word counting.  The BRX always branches back to location 5.

This change allows a true negative word count to be placed in location 11 to work with a WIM XXXXX,2 where XXXXX is one location past the region to be loaded by the boot sequence.  XXXXX plus the negative count in location 11 (buf[3]+buf[7]) initially points to the first location to be loaded, and the BRX will not branch when the word count is exhausted (register X=0) allowing the branch to the start address to be executed.

Old allowable boot sequence:
location  2 = WIM 12,2
location  3 = BRX 2
location  4 = LDX 11
location  5 = WIM 0,2
location  6 = SKS 21000
location  7 = BRX 5
location 10 = BRU Transfer Address
location 11 = Load address with bit 9 set to force BRX to branch

New allowable boot sequence:
location  2 = WIM 12,2
location  3 = BRX 2
location  4 = LDX 11
location  5 = WIM [end load address + 1],2
location  6 = SKS 21000
location  7 = BRX 5
location 10 = BRU Transfer Address
location 11 = Negative count to number of words to load
PDP11, VAX: Fix differencing virtual disk corruption during creation problems
PDP11, VAX: Fix failure to create differencing VHDs when the parent VHD is larger than 2GB.
PDP11, VAX: Fix VHD Differencing disk parent locator descriptors and removed deprecated descriptor types
PDP10, PDP11, VAX: Avoid corrupting a DMC/DMR unit attach string when a line state transitions
SDS: Allow console bootstrap from RAD if connected to W channel

Normally the RAD is connected to the E channel and there is no bootstrap capability (because of hardwired WIM instruction). However, for completeness, add code to allow booting from the RAD if it is connected to the W channel. Add RAD_CHAN definition to define which controller the RAD is connected to.

Improve parentheses in bootstrap code that allows an optional word count (while also permitting the canonical bootstrap sequence).
Merge git://github.com/simh/simh
SDS: Further refinement to bootstrap from RAD, update SDS documentation

Instead of conditionally compiling in bootstrap support depending upon attached channel, determine dynamically based on RAD's current assignment to channel W or channel E.  This allows using "set rad channel=W" for purposes of bootstrapping, and then relocating the RAD with "set rad channel=E" for running a TSS monitor that expects to find the RAD on channel E.
PDP11, VAX: Avoid strcpy use on overlapping buffers when creating and referencing VHDs.
PDP10, PDP11, VAX: Fix DUP and DMC devices which couldn't be disabled after having been enabled.