Added support for a new break type 'D' which acts as a watchpoint.  If
you do a 'br -D <some-loc>', the value at that location will be reported
every time the location is read or written, but emulation will not stop.

Initial support for displaying the type of PL/1 variables in debug_io.cpp.

Fix where IOM would see ~0 instead of zero when using FEATMEM_CHECK_UNINIT.

Use stock SIMH mechanisms to display the names of the IR register flags
instead of using a custom mechanism.

Add a unit to the opcon so that SIMH will print a newline after displaying
the opcon state.

Display the values written by the bootload channel.

Better detection of DIS (delay until interrupt) with no activities
pending (by ignoring queued activities for the internal console
checks).

Allow debugging of the loading of PL/1 sources by listing.cpp