This continues the previous commit, in which VAR_GLOBAL, VAR_INTERNAL
and VAR_CMDLINE were renamed.
Renaming the variable 'ctxt' was trivial since that word is used nowhere
else. In the comments though, each occurrence of the word 'context' had
to be checked individually since the word 'context' was not only used
for referring to a variable scope. It is also used to distinguish
different situations where characters are escaped in a certain way
('parsing context') and in a few other expressions.
The error marker var_Error is just an empty string. In the debug log
this empty string was not distinguishable from an otherwise empty
string.
Having this distinction helps in understanding the exact data flow.
