Deprecate uses of POSIX::isfoo()

Deprecate uses of POSIX::isfoo()

These functions have been supplanted in more modern Perls by
/[[:posix:]]/.  The documentation has been wrong; they don't handle
UTF-8 and return true on an empty string.  Rather than try to fix them,
the decision has been made to deprecate them instead.

See http://markmail.org/thread/jhqcag5njmx7jpyu

This commit also updates the documentation to be accurate.