librspreload: Make socket_fallback() call more generic

librspreload: Make socket_fallback() call more generic

socket_fallback is used to switch from an rsocket to a normal
socket in the case of failures.  Rename the call and make it
more generic, so that it can switch between an rsocket and
a normal socket in either direction.  This will be used to
support fork().

As part of this change, we move the list of hooked and rsocket
calls into structures, versus maintaining a large number of
static variables.

Signed-off-by: Sean Hefty <sean.hefty@intel.com>