ipc: more error-checking for sendmsg(..., MSG_EOR)

ipc: more error-checking for sendmsg(..., MSG_EOR)

Introduce `sendmsg_eor' for error checking to ensure we don't hit
short writes when writing to SOCK_SEQPACKET sockets.  Reducing
imports of MSG_EOR will also make it easier for us to drop it in
the future if certain OSes end up being too buggy with it (as was
the case with OpenBSD 7.3).  It'll also make it easier to switch
to SOCK_DGRAM in the future if necessary, since Unix datagram
sockets seem reliable in practice.