C++20 port 3/14: compile crypto, config, DNS, ACL and plugin modules as C++ (#3067)

* Make the crypto, config, DNS and plugin modules compile as C++

Cast allocator and memchr results explicitly, pass mbedtls_operation_t
to mbedtls_cipher_setkey, and declare the two locals in
ss2022_encrypt_first() before the gotos that jump past them. Drop the
unused RESOLV_MODE global that resolv.c defined by accident through an
anonymous enum declaration. Still valid C.

* Raise the test origin's listen backlog

socketserver listens with a backlog of 5. With concurrent connections
through the proxy on a loaded machine (parallel ctest), the backlog
overflows, the kernel resets the origin connection, and the proxy
correctly drops the client, which surfaced as intermittent
alloc_audit_report failures ("getpeername: Invalid argument" in
ss-local, "Connection reset by peer" in ss-server). Reproduced
reliably at 64 concurrent connections; gone with a backlog of 256.

* Compile the crypto, config, DNS, ACL and plugin modules as C++

Rename netutils, jconf, stream, aead, aead2022, aead2022_udp, crypto,
acl, plugin, plugin_disabled and resolv from .c to .cpp without changing
their contents, and update source lists, snippet references, test
comments and debian/copyright.

The Doxygen source list and scripts/check_cli_docs.py now accept either
extension for each module, so the remaining programs can move one at a
time. The new .cpp files are added to the no-heap ratchet baseline.

scripts/ss-setup.sh

@@ -14,7 +14,7 @@ readonly CONFIG_DIR="/etc/shadowsocks-libev"
 readonly SYSTEMD_UNIT_DIR="/etc/systemd/system"
 readonly SYSTEMD_TEMPLATE="shadowsocks-libev-server@.service"
 
-# AEAD ciphers supported by shadowsocks-libev (from src/aead.c)
+# AEAD ciphers supported by shadowsocks-libev (from src/aead.cpp)
 readonly AEAD_CIPHERS=(
     "chacha20-ietf-poly1305"
     "aes-256-gcm"