Move netmgr/uv-compat.h to <isc/uv.h>
As we are going to use libuv outside of the netmgr, we need the shims to be readily available for the rest of the codebase.
Move the "netmgr/uv-compat.h" to <isc/uv.h> and netmgr/uv-compat.c to uv.c, and as a rule of thumb, the users of libuv should include <isc/uv.h> instead of <uv.h> directly.