Simplified net::mux_conn interface

The recv_some function now takes only two parameters: a pointer and a size. This
eliminates the cap and offset parameters. The function no longer returns a
value.

The alloc_ibuffer and dealloc_ibuffer have been replaced with acquire_ibuffer
and release_ibuffer, respectively.

The overall intention of these changes is to simplify input buffer ownership.
The default implementation of the mux_conn class is for the mux_conn base class
to own the input buffer, which means the class is responsible for freeing the
buffer. However, derived classes may override the acquire_ibuffer and
release_ibuffer functions to take ownership of the buffer, in which case the
derived class owns the buffer for the lifetime of the buffer. In short, the new
interface eliminates the old strategy of transferring ownership and uses instead
a fixed-ownership scheme, whereby _either_ the base class _or_ the derived class
owns a buffer.