[[feature-stackdir]] Add MCStackdirStringIsUUID().

Add a function that tests whether a string is a properly-formatted
UUID in hyphenated hexadecimal format.
[[feature-stackdir]] Properly check UUIDs while saving.
[[feature-stackdir]] MCStackdirExecInternalSave(): Use MCAutoArrayRef.
[[feature-stackdir]] Implement MCStackdirIONewLoad().
[[feature-stackdir]] Implement MCStackdirIOSetConflictPermitted().
[[feature-stackdir]] Add MCStackdirIOGetConflictPermitted().
[[feature-stackdir]] Provide stub for MCStackdirIOHasConflict().

At the moment, this always returns false; the conflict detection and
handling support will come later.
[[feature-stackdir]] Implement MCStackdirIOGetState().
[[feature-stackdir]] Add "_internal stackdir load" debugging command.
[[feature-stackdir]] Resolve error location filenames when set.

If we don't resolve them when they are set, problems occur if the
current working directory changes.