We already call it that way internally.

If BASHUM_CTXSYM is set, show it in a different color.  This is intended
for special-purpose Bash invocations with modified environment, for example
when authentication to some remote service has been pre-set.

Finally stop using few-low-lettered globals; use proper Shellfu compliant
naming instead.

Also have separate variables for use in Bash PS*; these need extra chars
for hinting.

*   More intuitive file naming,
*   setup simplified,
*   setup is now in script (vs. diff file),
*   removed unnecessary exec mode.