TAPE: Update tape gap support (Dave Bryan)

With this update, the erase gap operation has been split out of
"sim_tape_wrgap" into a separate, internal "tape_erase_fwd" routine that
is called from "sim_tape_wrgap" as well as from the new "sim_tape_errecf"
routine.  There's a corresponding internal "tape_erase_rev" that's called
from the new "sim_tape_errecr" routine.

I've shimmed "sim_tape_rdlntf" and "sim_tape_rdlntr" to move the tape
context and debug stuff out of the routines that I'm maintaining.  This
will allow me to replace those functions in their entirety with the
corresponding functions in my development sources for future updates.
It also allows me to keep Bob's version in sync.  As my routines are static
and only called once from the shims, compilers should optimize away
the function calls and instead inline the code, so there'd be no extra call
overhead.

I'd also like to keep "tape_erase_fwd" and "tape_erase_rev" untouched for
the same reason.  If you wish to add debug calls to "sim_tape_errecf" and
"sim_tape_errecr", that's fine.

This commit is contained in: