Add FFI bindings for libcurl's "multi" interface

[rdw-software]

Separate tracking issue (mostly for testing GitHub's new sub-issues feature...).

Bindings coverage:

• curl_multi_add_handle
• curl_multi_assign
• curl_multi_cleanup
• curl_multi_fdset
• curl_multi_get_handles
• curl_multi_info_read
• curl_multi_init
• curl_multi_perform
• curl_multi_poll
• curl_multi_remove_handle
• curl_multi_setopt
• curl_multi_socket
• curl_multi_socket_action
• curl_multi_socket_all
• curl_multi_strerror
• curl_multi_timeout
• curl_multi_wait
• curl_multi_waitfds
• curl_multi_wakeup

See https://curl.se/libcurl/c/libcurl-multi.html

[rdw-software]

Integration with libuv may involve a few extra steps: https://curl.se/libcurl/c/multi-uv.html (Details are TBD)

Off the top of my head: Win32 socket descriptor initialization (curl has a flag to control this), cleanup, luv/uws interop.