Remove unused config entry "network_desc" (Network Description)
channel: Stop pretending to support per-mode server capabilities.

After a change for dynamic server capabilities, the code to send out mode
changes was changed to use the capabilities belonging to the last mode
being sent out. This does not make sense; therefore, just use no
capabilities and remove supporting infrastructure.
channel: remove write-only field.
Allow building Charybdis with a shared SQLite3 library instead of the in-tree .c file