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.