net-im/prosody: Do not use prosodyctl in init script

• Apply shebangfix.
• Update init script to directly control the Prosody daemon as using prosodyctl has been deprecated. [1]
• Add pkg-message to clearly state that setting the pidfile option in prosody.cfg.lua is required.
• Fix pkg-message formatting.

[1] https://blog.prosody.im/prosody-0.11.6-released/

PR: 249276
Submitted by: Thomas Morper <thomas@beingboiled.info> (maintainer)