refactor: modernize deprecated ipc headers #32781

Split off from #31802.

Pre-empt tidy warning when multiprocess is on by default:

[10:33:51.654] /ci_container_base/src/ipc/capnp/protocol.cpp:20:10: error: inclusion of deprecated C++ header 'errno.h'; consider using 'cerrno' instead [modernize-deprecated-headers,-warnings-as-errors]
[10:33:51.654]    20 | #include <errno.h>
[10:33:51.654]       |          ^~~~~~~~~
[10:33:51.654]       |          <cerrno>
[10:33:51.654] 919 warnings generated.

https://github.com/bitcoin/bitcoin/pull/31802/checks?check_run_id=43968493627

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For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/32781.

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lgtm ACK 74b7e9c7dbf73b6df84ba8a1f40dd22924a604ef

Looks good to me

ACK

Code review ACK 74b7e9c7dbf73b6df84ba8a1f40dd22924a604ef