Change the signature of uv_buf_init() from (char*, unsigned int) to
(char*, size_t). This is to accommodate the fact that the "len" property
of uv_buf_t is a size_t, and fixes a problem where "len" is truncated
for large "base".
Fixes: https://github.com/libuv/libuv/issues/1233
Refs: https://github.com/libuv/libuv/pull/1236
PR-URL: https://github.com/libuv/libuv/pull/1806
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
