tcptls: Don't inhibit escalations for outbound client connections.

tcptls: Don't inhibit escalations for outbound client connections.

handle_tcptls_connection() marks the current thread as inhibiting
privilege escalations and as an external user interface so that
dialplan functions considered 'dangerous' (STAT, SHELL, ...) cannot be
executed on behalf of an external protocol.

This is correct for inbound (server) connections, each of which runs on
its own dedicated worker thread. However, the outbound (client) path
calls handle_tcptls_connection() synchronously on the caller's own
thread. When that caller is a channel/PBX thread, the thread-local
flags are set and never cleared, permanently tainting the dialplan
thread.

Examples:

- Calling Dial() for an outbound WebSocket and resuming dialplan
  execution with the "g" flag.
- Running ExternalIVR() then continuing dialplan executing.

Outbound connections are initiated by Asterisk itself and are not
external user interfaces, so the flags should not be set for them. Gate
the flag-setting on tcptls_session->client so it applies only to inbound
connections.

Resolves: #2038