Add Cvar::ClearFlags; make it and Cvar::AddFlags return true if the cvar exists.
Implement ‘user archive’ flag for cvars; always save archived variables.

This partially reverts commit 0e8b0ed53a9155134286fca0beff88623f6476b6.

seta now sets USER_ARCHIVE. ARCHIVE is no longer user-settable.
Implement “set -[no-]archive” and “reset [-value]”.
Make /reset accept more than one variable name.
Add /archive and /unarchive for user archiving of cvars.