Protocols.DNS complains when an ephemeral port fails to open with EACCES.

Imported from https://youtrack.roxen.com/issue/PIKE-185

Reported by @grubba

Running test packages/sitebuilder/test/tests/RoxenTest_acauth_cookie.pike
Protocols.DNS: Binding of UDP port failed with errno 13: Permission denied
Internal server error: ################ Background failure
Stdio.UDP->bind: failed to bind to port 2049
-:1: Stdio.UDP()->bind(2049,"::",1)
pike/lib/modules/Protocols.pmod/DNS.pmod:267: Protocols.DNS.safe_bind(Stdio.UDP(),2049,"::")

EACCES indicates that the port is privileged.