chanserv/flags: make Anope FLAGS compatibility an option

chanserv/flags: make Anope FLAGS compatibility an option

Previously, ChanServ FLAGS behavior could be modified by registering or
dropping the keyword nicks "LIST", "CLEAR", and "MODIFY".
Now, a configuration option is available that when turned on (default),
disables registration of these keyword nicks and enables this
compatibility feature. When turned off, registration of these keyword
nicks is possible, and compatibility to Anope's FLAGS command is
disabled.

Fixes atheme/atheme#397