This is not needed but it does not cost much to do it for all
potentially supported format in libarchive. XZ and LZMA are not
builtin libarchive and require external lib but if
CMAKE_USE_SYSTEM_LIBARCHIVE is ON then we may get it for free.
This will be needed to use cmArchiveWrire in cmCPackArchiveGenerator
with the same feature set as before. Note that adding zip
support to libarchive-wrapper would also makes it easy to add
a new -E zip command to cmake commands.
Some stream libraries return size_t from gcount() and some return
ssize_t. Add an explicit cast to ios::streamsize for its return value.
Also refactor use of nnext to reduce the use of casts.
The libarchive interface accepts size_t but returns ssize_t. The std
streams interface wants streamsize, which is typically ssize_t. Since
no one type for our variable matches without conversions, make the
conversions explicit to avoid -Wsign-conversion and -Wsign-compare
warnings.
