src: use std::string for trace enabled_categories

A std::string manages its own memory, so using one removes the implicit
assumption that the argv vector passed to node will never be
deallocated. Also, the enabled_categories are used to construct a
std::stringstream, so its simpler to use the standard library
consistently.

PR-URL: https://github.com/nodejs/node/pull/12242


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>