[PHP] fix: raise SIGPIPE for bridged TCP EPIPE

[brandonpayton]

Why

POSIX requires that writing to a stream socket whose peer has closed the read end returns EPIPE and raises SIGPIPE, unless the caller opts out with MSG_NOSIGNAL (or SO_NOSIGPIPE). Kandelo's in-kernel pipe-backed sockets already did this, but the host-bridged TCP path (host_net_send for AF_INET/AF_INET6) only propagated the EPIPE errno and never raised the signal — and it did not honor MSG_NOSIGNAL. So PHP stream/socket tests saw non-Unix behavior that depended on whether a connection was loopback or host-bridged.

Making the bridged path match the in-kernel socket model is a platform fix that benefits every package using bridged TCP, not just PHP.

What

• crates/kernel/src/syscalls.rs: in sys_write's external-TCP branch, map an EPIPE from host_net_send to raising SIGPIPE (mirroring the pipe-backed path). In sys_send, honor MSG_NOSIGNAL — suppress the SIGPIPE a write would raise, and on the external path raise SIGPIPE on EPIPE only when MSG_NOSIGNAL is unset.
• Extends MockHostIO so host_net_connect / connect_status / send / listen results are configurable, and adds tests: external-TCP EPIPE raises SIGPIPE; send(..., MSG_NOSIGNAL) suppresses it.

Provenance

Supersedes fork-headed PR #729; identical head commit 9e252cfb8d6af2e1dd5c91d61c1357456411b84d, re-pointed onto an Automattic/kandelo branch. No code changed from #729.