Implement TLS support using Botan

For now, it tries two TLS ports and then connects to the non-tls port.  In
the future we would like the user to be able to configure that.

fix #2435

cmake/Modules/FindBotan.cmake

@@ -0,0 +1,34 @@
+# - Find botan
+# Find the botan cryptographic library
+#
+# This module defines the following variables:
+#   BOTAN_FOUND  -  True if library and include directory are found
+# If set to TRUE, the following are also defined:
+#   BOTAN_INCLUDE_DIRS  -  The directory where to find the header file
+#   BOTAN_LIBRARIES  -  Where to find the library file
+#
+# For conveniance, these variables are also set. They have the same values
+# than the variables above.  The user can thus choose his/her prefered way
+# to write them.
+#   BOTAN_LIBRARY
+#   BOTAN_INCLUDE_DIR
+#
+# This file is in the public domain
+
+find_path(BOTAN_INCLUDE_DIRS NAMES botan/botan.h
+  DOC "The botan include directory")
+
+find_library(BOTAN_LIBRARIES NAMES botan
+  DOC "The botan library")
+
+# Use some standard module to handle the QUIETLY and REQUIRED arguments, and
+# set BOTAN_FOUND to TRUE if these two variables are set.
+include(FindPackageHandleStandardArgs)
+find_package_handle_standard_args(Botan REQUIRED_VARS BOTAN_LIBRARIES BOTAN_INCLUDE_DIRS)
+
+if(BOTAN_FOUND)
+  set(BOTAN_LIBRARY ${BOTAN_LIBRARIES})
+  set(BOTAN_INCLUDE_DIR ${BOTAN_INCLUDE_DIRS})
+endif()
+
+mark_as_advanced(BOTAN_INCLUDE_DIRS BOTAN_LIBRARIES)