Move get_cont_octet_out_count_impl from unnamed namespace to namespace detail. Workaround for unnamed namespace symbols having external linkage prior to C++11, which causes duplicate symbol errors in libraries (for example filesystem) that link in multiple object files (differing only in namespaces) built from the same source file, allowing two versions to exist side-by-side in the same lib or dll.
Fix #10354 from Juan Alday. As he pointed out, the declaration (in detail/utf8_codecvt_facet.hpp) and definition (in detail/utf8_codecvt_facet.ipp) need to use the same signature.  Reapply Marshall Clow's [SVN r81616] and [SVN r81877] from November and December, 2012, as these appear to correctly supply const for the msvc/dinkumware library (since fixed) that used a do_length() first argument. Revert the portion of [SVN r86722] that removed BOOST_CODECVT_DO_LENGTH_CONST from the do_length() signature in utf8_codecvt_facet.ipp so that it stays in sync with utf8_codecvt_facet.hpp. For the same reason, copy the __IBMCPP__ workaround from utf8_codecvt_facet.ipp to utf8_codecvt_facet.hpp. Verified all libraries (filesystem, log, program_options, property_tree, serialization) that use utf8_codecvt_facet passed all tests before, and then after, these changes. Also applied minor edits to the use comments embedded in utf8_codecvt_facet.hpp.
eliminated MACRO for do length by providing both const and non-const versions