Send piler's syslog output to stderr

[stephdl]

Stacked on #31, itself stacked on #30. Retarget as those merge.

Problem

piler and piler-smtp log exclusively through syslog(3). There is no syslog daemon in this image and no /dev/log socket — /dev is a root-owned tmpfs created by the runtime, and the container is uid 1000 with cap_drop: ALL, so nothing can create one. Every line the two daemons produced was silently discarded. They were the only things in the container without logs, while nginx and php-fpm stream to stdout/stderr.

This is not theoretical: diagnosing a failure during #30 required injecting a syslog collector with podman exec --user root to see anything at all.

Change

config/syslog-to-stderr.c is a ~40-line LD_PRELOAD shim that overrides openlog, syslog, vsyslog, __syslog_chk and __vsyslog_chk and writes ident[pid]: message to stderr, expanding %m the way glibc does. __syslog_chk is the one that matters — the released binaries are built fortified:

$ nm -D --undefined-only /usr/sbin/piler | grep -i syslog
                 U __syslog_chk@GLIBC_2.4

It is compiled in its own build stage (gcc and libc6-dev never reach the runtime image) and preloaded for piler, piler-smtp and supercronic only — not for php-fpm or nginx, which have real log configuration.

Upstream is the better place to fix this: LOG_PERROR when not daemonising, or a config toggle. Since #30 both daemons run in the foreground, so that flag alone would do it. The shim can be dropped if that lands.

Validation

Built and run through the full compose stack. Before this change podman logs piler had no piler lines at all; now:

piler-smtp[62]: piler-smtp 1.4.9-d3f4b07 starting
piler[96]: piler 1.4.9-d3f4b07 starting
piler-smtp[62]: received: HGSCNS7IQOF4EMLY, from=s@example.com, size=95, client=10.89.0.5, fd=5, fsync=4121
piler[97]: 0/HGSCNS7IQOF4EMLY: 5000...fd61, size=95/112, attachments=0, message-id=<shim-1@example.com>, status=stored
piler-smtp[62]: piler-smtp has been terminated
piler[96]: piler has been terminated

Also re-ran #31's drain scenario under the shim: 200 mails sent, stop -t 60 logged draining spool, 34 file(s) left then spool drained, and all 200 were archived. Compiles clean with -Wall -Wextra.