Add support for FocusPosition in Sony RAW files (#906)

* Fix 582 Add support for FocusPosition in Sony RAW files
* Thanks to @boardhead sonyFpCrypt() works correctly. Removed debug code. Fixed typos.
* Update doc/templates/Makefile to process Sony2Fp
* Following review by @boardhead. Renamed sonyFpCrypt() as sonyTagDecipher().
* Fixed writing the tag thanks to @boardhead explaining encipher/decipher.
  Sadly, ArrayCfg/crpyt does not know if he's encrypting/decrypting.
  I've added a sniff in TiffEncoder::visitBinaryArrayEnd to avoid changing the API.
* Added URL to discussion concerning sonyTagCipher()
* make sonyTagCipher() a static function with no external visibility.

tests/bugfixes/github/test_pr_906.py

@@ -0,0 +1,17 @@
+# -*- coding: utf-8 -*-
+
+from system_tests import CaseMeta, path
+
+class Sony2FpTest(metaclass=CaseMeta):
+
+    filename = path("$data_path/exiv2-pr906.exv")
+    commands = ["$exiv2 -pa --grep Sony2Fp $filename"]
+
+    stdout = ["""Exif.Sony2Fp.AmbientTemperature              SByte       1  19
+Exif.Sony2Fp.FocusMode                       Byte        1  6
+Exif.Sony2Fp.AFAreaMode                      Byte        1  12
+Exif.Sony2Fp.FocusPosition2                  Byte        1  140
+"""
+    ]
+    stderr = [""]
+    retval = [0]