[dovecot/core] bd2a17: quota: Add backend register/unregister

[GitHub]

  Branch: refs/heads/master
  Home:   https://github.com/dovecot/core
  Commit: bd2a176b573f9679e7e45339c20ef71704f071c0
      https://github.com/dovecot/core/commit/bd2a176b573f9679e7e45339c20ef71704f071c0
  Author: Aki Tuomi <aki.tuomi at dovecot.fi>
  Date:   2017-03-26 (Sun, 26 Mar 2017)

  Changed paths:
    M src/plugins/quota/quota-plugin.c
    M src/plugins/quota/quota-private.h
    M src/plugins/quota/quota.c

  Log Message:
  -----------
  quota: Add backend register/unregister

This way, other mail plugins can register their own
quota backends.