core_user Inherit CommandNick from SplitCommand, only handle it for local users
core_user Deduplicate code that calls the OnUserRegister hook
Move calling the OnUserPreNick() hook and the restrictbannedusers check from core to cmd_nick (core_user)
Remove now needless User::ForceNickChange()

Change call sites to call ChangeNick()
Deduplicate code in User::ChangeNick()
Get rid of irc::strings (assign()) in User::ChangeNick()