unix: change error code for recvmsg() failed due to RLIMIT_NOFILE

Instead of returning EMSGSIZE pass the error code from fdallocn() directly
to userland. That would be EMFILE, which makes much more sense. This
error code is not listed in the specification[1], but the specification
doesn't cover such edge case at all. Meanwhile the specification lists
EMSGSIZE as the error code for invalid value of msg_iovlen, and FreeBSD
follows that, see sys_recmsg(). Differentiating these two cases will make
a developer/admin life much easier when debugging.

[1] https://pubs.opengroup.org/onlinepubs/9699919799/functions/recvmsg.html

Reviewed by: markj
Differential revision: https://reviews.freebsd.org/D35640