Convert RTP/RTCP/OSMUX I/O from osmo_fd to osmo_io

Converting from osmo_fd to osmo_io allows us to switch to the new
io_uring backend and benefit from related performance benefits.

In a benchmark running 200 concurrent bi-directional voice calls with
GSM-EFR codec, I am observing:

* the code before this patch uses 40..42% of a single core on a
  Ryzen 5950X at 200 calls (=> 200 endpoints with each two connections)

* no increase in CPU utilization before/after this patch, i.e. the
  osmo_io overhead for the osmo_fd backend is insignificant compared
  to the direct osmo_fd mode before

* an almost exactly 50% reduction of CPU utilization when running the
  same osmo-mgw build with LIBOSMO_IO_BACKEND=IO_URING - top shows
  19..21% for the same workload instead of 40..42% with the OSMO_FD
  default backend.

* An increase of about 4 Megabytes in both RSS and VIRT size when
  enabling the OSMO_IO backend.  This is likely the memory-mapped rings.

No memory leakage is observed when using either of the backends.

Change-Id: I8471960d5d8088a70cf105f2f40dfa5d5458169a
---
M include/osmocom/mgcp/mgcp.h
M include/osmocom/mgcp/mgcp_network.h
M src/libosmo-mgcp/mgcp_conn.c
M src/libosmo-mgcp/mgcp_iuup.c
M src/libosmo-mgcp/mgcp_network.c
M src/libosmo-mgcp/mgcp_osmux.c
M tests/mgcp/mgcp_test.c
7 files changed, 232 insertions(+), 169 deletions(-)