Core: Add support for systemd socket activation.

This change adds support for socket activation of certain SOCK_STREAM
listeners in Asterisk:
* AMI / AMI over TLS
* CLI
* HTTP / HTTPS

Example systemd units are provided.  This support extends to any socket
which is initialized using ast_tcptls_server_start, so any unknown
modules using this function will support socket activation.

Asterisk continues to function as normal if socket activation is not
enabled or if systemd development headers are not available during
build.

ASTERISK-27063 #close

Change-Id: Id814ee6a892f4b80d018365c8ad8d89063474f4d