Changes

bts-trx: Convert TRXC and TRXD sockets to iofd

Since now the Tx side is driven by the event loop, we can use (and
should) OSMO_SOCK_F_NONBLOCK on the socket, avoiding potential
blocking of the entire process.

We also gain io_uring support for free, which is a really nice feature
to have in TRXD.

Related: OS#1579
Change-Id: I239f91efad43eabd280caf9f852c3aefbc729eaf