mod_sed: review fixes, plus a test suite

[notroj]

Commentary from Claude:

A review of mod_sed and the sed/regexp code it bundles, with a test/modules/filters suite to go with it.

The response filter change is the one worth a close look. On a failed evaluation it skipped both the final flush and the ap_pass_brigade(), taking the EOS with them, so the header filters never ran and the client was left on a response that never finished. It now separates a failure before anything has been written, where the caller can still turn it into an error response and does, from one after, where the response is already on the wire and has to be ended as the truncated one it has become.

The remaining fixes are independent of each other: array handling when filling eval->abuf, l deriving its octal escapes from a sign-extended char, eval_errf() taking a format string from a runtime value, and a starred backreference to a capture which matched nothing, where neither loop advances the input pointer.

test/pyhttpd/env.py gains add_cgi_module() so a suite can ask for CGI without naming mod_cgid, and the win32-only cgid→cgi substitution now applies everywhere, so a build without mod_cgid falls back rather than failing. docs/manual/mod/mod_sed.xml drops .htaccess from both directives' context lists, since they are ACCESS_CONF.

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