Add -z switch (no initial window references) to encoder

[ecm-pushbx]

As discussed on the SvarDOS issue tracker, refer to: SvarDOS/bugz#127 (comment)

Particular the store zeros calls concern me because they may impose complications on the depacker, if it doesn't use a circular buffer. I may look into adding a mode to heatshrink that forbids references that point before the depacked data.

For example, the lDOS inicomp depacker for heatshrink does some special handling to store zeros. This code can be dropped if the encoder's new -z switch is used:
https://hg.pushbx.org/ecm/inicomp/file/f5f72d7078a2/heatshr.asm#l443

[ecm-pushbx]

I previously discussed my 8086 depackers in #82

[ecm-pushbx]

I updated both the inicomp executable depacker and the lDebug help page depacker to make use of an optional define known as _Z. With this set, the zero storing special handling is excluded. I also updated the lDebug and kernwrap mak.sh scripts to detect and make use of the Z mode, passing -z to the encoder.