core: Add common apr_off_t config parser

[arturobernalg]

Follow-up to the parsing discussion in #825.

Adds a common helper, ap_off_parameter_parse(), for converting configuration input to an apr_off_t, analogous to the existing timeout parser. It accepts a whole decimal number (as apr_strtoff() does) and fails on trailing characters, an empty value and overflow. It returns APR_EINVAL for a malformed number and APR_ERANGE for overflow, and only sets the value on success. The range of the value stays with each directive.

It converts the existing callers where the value is an apr_off_t or fits in one, and keeps each directive's own range checks (narrowing to long, int or apr_size_t is checked before the cast):

• LimitRequestBody (no change in behavior or messages, checked by comparing the accepted values before and after)
• CacheMinFileSize, CacheMaxFileSize, CacheReadSize, CacheReadTime (mod_cache_disk)
• CacheSocacheMaxSize, CacheSocacheMaxTime, CacheSocacheMinTime, CacheSocacheReadSize, CacheSocacheReadTime
• BufferSize, AuthFormSize
• DavQuota
• ScriptLogLength, ScriptLogBuffer (mod_cgi and mod_cgid)

This also makes malformed values such as 123abc fail at startup consistently instead of being partially accepted. Most of these directives used apr_strtoff() without checking the end of the number, and DavQuota, ScriptLogLength and ScriptLogBuffer used atol()/atoi(), so abc meant 0. Negative values of DavQuota (they also took the place of the Off/None values), ScriptLogLength and ScriptLogBuffer are now refused.

LimitXMLRequestBody is left to #825, and the directives with units (e.g. SubstituteMaxLineLength) are not touched.

The new function is exported API, so the MMN minor is bumped.

Tested with httpd -t against trunk and the patch for every directive above, and with new pyhttpd tests (core/cache/cgid, DavQuota, BufferSize). I also ran the pytest_suite cache, cgi, buffer, dav and limits tests, where nothing changed. The new tests were not run on Windows. mod_auth_form is only checked by that comparison, and the mod_cgi variant was only compiled.

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