res_rtp_asterisk: Add support for DTLS packet fragmentation.

This change adds support for larger TLS certificates by allowing
OpenSSL to fragment the DTLS packets according to the configured
MTU. By default this is set to 1200.

This is accomplished by implementing our own BIO method that
supports MTU querying. The configured MTU is returned to OpenSSL
which fragments the packet accordingly. When a packet is to be
sent it is done directly out the RTP instance.

ASTERISK-28018

Change-Id: If2d5032019a28ffd48f43e9e93ed71dbdbf39c06

configure

@@ -1129,6 +1129,10 @@ PBX_DAHDI
 DAHDI_DIR
 DAHDI_INCLUDE
 DAHDI_LIB
+PBX_OPENSSL_BIO_METHOD
+OPENSSL_BIO_METHOD_DIR
+OPENSSL_BIO_METHOD_INCLUDE
+OPENSSL_BIO_METHOD_LIB
 PBX_OPENSSL_SRTP
 OPENSSL_SRTP_DIR
 OPENSSL_SRTP_INCLUDE
@@ -9802,6 +9806,18 @@ PBX_OPENSSL_SRTP=0
 
 
 
+OPENSSL_BIO_METHOD_DESCRIP="OpenSSL BIO Method Support"
+OPENSSL_BIO_METHOD_OPTION=crypto
+OPENSSL_BIO_METHOD_DIR=${CRYPTO_DIR}
+
+PBX_OPENSSL_BIO_METHOD=0
+
+
+
+
+
+
+
     DAHDI_DESCRIP="DAHDI"
     DAHDI_OPTION="dahdi"
     PBX_DAHDI=0
@@ -30719,6 +30735,102 @@ _ACEOF
 fi
 
 
+
+if test "x${PBX_OPENSSL_BIO_METHOD}" != "x1" -a "${USE_OPENSSL_BIO_METHOD}" != "no"; then
+   pbxlibdir=""
+   # if --with-OPENSSL_BIO_METHOD=DIR has been specified, use it.
+   if test "x${OPENSSL_BIO_METHOD_DIR}" != "x"; then
+      if test -d ${OPENSSL_BIO_METHOD_DIR}/lib; then
+         pbxlibdir="-L${OPENSSL_BIO_METHOD_DIR}/lib"
+      else
+         pbxlibdir="-L${OPENSSL_BIO_METHOD_DIR}"
+      fi
+   fi
+
+      ast_ext_lib_check_save_CFLAGS="${CFLAGS}"
+      CFLAGS="${CFLAGS} "
+      { $as_echo "$as_me:${as_lineno-$LINENO}: checking for BIO_meth_new in -lssl" >&5
+$as_echo_n "checking for BIO_meth_new in -lssl... " >&6; }
+if ${ac_cv_lib_ssl_BIO_meth_new+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+  ac_check_lib_save_LIBS=$LIBS
+LIBS="-lssl ${pbxlibdir} -lcrypto $LIBS"
+cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+
+/* Override any GCC internal prototype to avoid an error.
+   Use char because int might match the return type of a GCC
+   builtin and then its argument prototype would still apply.  */
+#ifdef __cplusplus
+extern "C"
+#endif
+char BIO_meth_new ();
+int
+main ()
+{
+return BIO_meth_new ();
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_c_try_link "$LINENO"; then :
+  ac_cv_lib_ssl_BIO_meth_new=yes
+else
+  ac_cv_lib_ssl_BIO_meth_new=no
+fi
+rm -f core conftest.err conftest.$ac_objext \
+    conftest$ac_exeext conftest.$ac_ext
+LIBS=$ac_check_lib_save_LIBS
+fi
+{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $ac_cv_lib_ssl_BIO_meth_new" >&5
+$as_echo "$ac_cv_lib_ssl_BIO_meth_new" >&6; }
+if test "x$ac_cv_lib_ssl_BIO_meth_new" = xyes; then :
+  AST_OPENSSL_BIO_METHOD_FOUND=yes
+else
+  AST_OPENSSL_BIO_METHOD_FOUND=no
+fi
+
+      CFLAGS="${ast_ext_lib_check_save_CFLAGS}"
+
+
+   # now check for the header.
+   if test "${AST_OPENSSL_BIO_METHOD_FOUND}" = "yes"; then
+      OPENSSL_BIO_METHOD_LIB="${pbxlibdir} -lssl -lcrypto"
+      # if --with-OPENSSL_BIO_METHOD=DIR has been specified, use it.
+      if test "x${OPENSSL_BIO_METHOD_DIR}" != "x"; then
+         OPENSSL_BIO_METHOD_INCLUDE="-I${OPENSSL_BIO_METHOD_DIR}/include"
+      fi
+      OPENSSL_BIO_METHOD_INCLUDE="${OPENSSL_BIO_METHOD_INCLUDE} "
+
+         # check for the header
+         ast_ext_lib_check_saved_CPPFLAGS="${CPPFLAGS}"
+         CPPFLAGS="${CPPFLAGS} ${OPENSSL_BIO_METHOD_INCLUDE}"
+         ac_fn_c_check_header_mongrel "$LINENO" "openssl/ssl.h" "ac_cv_header_openssl_ssl_h" "$ac_includes_default"
+if test "x$ac_cv_header_openssl_ssl_h" = xyes; then :
+  OPENSSL_BIO_METHOD_HEADER_FOUND=1
+else
+  OPENSSL_BIO_METHOD_HEADER_FOUND=0
+fi
+
+
+         CPPFLAGS="${ast_ext_lib_check_saved_CPPFLAGS}"
+
+      if test "x${OPENSSL_BIO_METHOD_HEADER_FOUND}" = "x0" ; then
+         OPENSSL_BIO_METHOD_LIB=""
+         OPENSSL_BIO_METHOD_INCLUDE=""
+      else
+
+         PBX_OPENSSL_BIO_METHOD=1
+         cat >>confdefs.h <<_ACEOF
+#define HAVE_OPENSSL_BIO_METHOD 1
+_ACEOF
+
+      fi
+   fi
+fi
+
+
 fi
 
 if test "$PBX_OPENSSL" = "1";