Use libuv's shared library handling capabilities

Use libuv's shared library handling capabilities

While libltdl is a feature-rich library, BIND 9 code only uses its basic
capabilities, which are also provided by libuv and which BIND 9 already
uses for other purposes.  As libuv's cross-platform shared library
handling interface is modeled after the POSIX dlopen() interface,
converting code using the latter to the former is simple.  Replace
libltdl function calls with their libuv counterparts, refactoring the
code as necessary.  Remove all use of libltdl from the BIND 9 source
tree.