Correct name, class name & file name for tracevar
Correct tracevar main class comment
Tracevar not allowed in post_mortem mode
Remove unnecessary to_sym
Use real global variable names (with $)
Add "untracevar" command for completeness
Remove "trace" for post_mortem tests

That command does not exist anymore. The test was only passing because
"trace" is taken as an abbreviation for "tracevar"
Don't define global variables

It's kind of encouraging a bad practice, so let's just use the variables
predefined in Ruby. It also removes some warnings of undefined
variables.