quota: Add and use QUOTA_USER_CONTEXT_REQUIRE Satisfies static analyzers

quota: Use mailbox_list_created hook to override list methods.

lib-storage: Plugin API changed to run plugin functions in correct order. Previously the hooks were run in correct order, but the functions they overrode were run in reverse order. This caused problems when multiple plugins were used. --HG-- branch : HEAD

Added doveadm quota plugin. --HG-- branch : HEAD

plugins: Simplified code by using mailbox_allocated hook. --HG-- branch : HEAD

plugins: Use mail_namespace_storage_added hook instead of mailbox_list_created. In mailbox_list_created the storage hasn't yet overridden the list functions. --HG-- branch : HEAD

lib-storage: Plugin hook API changed to support per-user plugins within a process. --HG-- branch : HEAD

Implemented support for per-namespace quotas. Can be used with public namespaces. --HG-- branch : HEAD

Fixed plugins to work with config rewrite. --HG-- branch : HEAD

Added struct mail_user and fixed the code to support multiple users per process. --HG-- branch : HEAD

Changed .h ifdef/defines to use <NAME>_H format. --HG-- branch : HEAD

Moved delete/rename operations to mailbox_list API. Fixed mbox/maildir to work with either fs/maildir++ directory layout. They can be changed by appending :LAYOUT=fs|maildir++ to mail_location. --HG-- branch : HEAD

s/quota/quota_set/ so we don't conflict with the quota symbol already present in Mac OS X. --HG-- branch : HEAD

Added beginnings of plugin infrastructure. TODO: These could be optionally compiled into binaries with some configure options. Added quota plugin and a new trash plugin. Not very well tested. --HG-- branch : HEAD