Allow for default umodes to change at runtime

[skizzerz]

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When default umodes are changed in ircd.conf, these changes will now be applied to all local clients upon receiving a /rehash. We track which umodes a user has explicitly (un)set and avoid modifying those umodes when changing the defaults -- only umodes that a user didn't explicitly touch are subject to change on rehash.

[edk0]

Hmm. It'd be nice if setting umode -R when you're already -R would mark R as a changed umode, but clearly harder to do that with how user_mode works now.

[skizzerz]

@edk0 Wasn't significantly harder to do, so I added that feature in as well.

[edk0]

This currently sets people -Z when the default umodes change