Allow eggdrop to handle SIGHUP signal

Summary:
Eggdrop rehashes on SIGHUP, but we need to disconnect from MySQL first.

Previously, a custom .rehash command existed, but there is a prerehash
event to put code used by AND .rehash AND the signal.

Ref T1323

Test Plan: Tested with Aeschere

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1323

Differential Revision: https://devcentral.nasqueron.org/D1942