These were never intended to be used outside _pretty_color; the fact
that _pretty_color is internal does not mean *its* internal globals do
not need to follow underscore rules.

Old naming is inconsistent from how functions are named, which can be
pretty confusing.

Also, there's no such naming requirement for user's modules; the
recommended way is to use unique project-specific prefix for both
functions and globals.

Risk of conflict should be managed by naming modules uniquely and/or
being careful with deployment in new environments.

Remove '()' decoration from $caller internal variable. Decorations are
matter of presentations; at this level the brackets are only confusing.

To decide whether a caller is main script or direct descendant thereof,
called main(), use negative index of the stack array FUNCNAME (0 in first
case, 1 in the second).  To signal that to the `_pretty_*` plug-ins,
use a dedicated boolean.

This also accounts for cases when a main() function is declared somewhere
else than in the main binary.  Older code would simply assume (based
on the name being "main") that it's called from main binary and rewrite
$caller, possibly causing confusion.  Now, only the originally intended
special cases behave specially.

Without -r, sequences like '\t' are translated, which we basically never
want.

A more conventional hierarchy

Ans its internal sub-modules to _pretty_*