Clean up tls-related feature flags

This provides a clear and consistent interface for selecting TLS-related
features on both (xmpp and tokio-xmpp) crates. All feature checks have
been revamped. All working combinations have been tested (including a
connectivity test + --all-features docs build) using:

```
set -xeuo pipefail
features=(aws_lc_rs ring ktls,aws_lc_rs ktls,ring aws_lc_rs,ring aws_lc_rs,ring,ktls native-tls rustls-any-backend)
export RUSTFLAGS=" -D warnings"
cargo test --no-default-features
cargo test
for feature in ${features[@]}; do
  echo ">>> BUILDING with $feature" 2>&1
  # Running code or building examples cannot succeed with rustls-any-backend.
  features="starttls,$feature"
  if [ "$feature" != 'rustls-any-backend' ]; then
    if ! cargo test --no-default-features --features="$features"; then
      echo ">>> BUILD FAILED for tls feature set: $features" >&2
      exit 1
    fi
    set +e
    timeout -sINT -p -k 2 3 cargo run --no-default-features --features="$features" --example keep_connection -- test@hub.sotecware.net "$(pass xmpp/test@hub.sotecware.net)"
    status="$?"
    set -e
    if [ $status -ne 0 ]; then
      echo ">>> keep_connection did not shut down cleanly! (status: $status)" >&2
      exit 1
    fi
  else
    if ! cargo build --no-default-features --features="$feature"; then
      echo ">>> BUILD FAILED for tls feature set: $features" >&2
      exit 1
    fi
  fi
done
RUSTDOCFLAGS="--cfg docsrs" RUSTFLAGS="--cfg xmpprs_doc_build" cargo +nightly doc -Zrustdoc-map --all-features
```

xmpp/ChangeLog

@@ -15,13 +15,14 @@ XXXX-YY-ZZ [ RELEASER <admin@localhost> ]
       - Agent::send_room_private_message now takes RoomPrivateMessageSettings (!487)
       - Event now exposes Option<MessageId> for incoming messages, and MessageId
         for incoming message corrections; type alias Id has been removed (!504)
-      - The `starttls-rust` feature flag does not automatically enable a
-        `rustls` crypto provider anymore. This is to avoid conflict between
-        two crypto providers and to avoid linking unnecessary code. The
-        `aws_lc_rs` crypto provider is still built by default, however,
-        applications which use `xmpp` without default features will have to
-        adapt their feature flags to explicitly enable the
-        `starttls-rust-aws_lc_rs` or `starttls-rust-ring` features. (!581)
+      - The TLS-related feature flags have been completely reworked to make
+        them easier to use. The `starttls-*` feature flags have been removed
+        in favour of more concise flag names identifying the TLS backends
+        directly (`aws_lc_rs`, `ring`, `native-tls`). The `starttls` feature
+        is now independent of the specific backend (but a backend still needs
+        to be enabled for compilation to succeed).
+
+        Please refer to the crate docs for details. (!581)
     * Added:
       - Agent::send_room_message takes RoomMessageSettings argument (!483)
       - Agent::send_raw_message takes RawMessageSettings for any message type (!487)