Fix typo spotted by Mike Rumph

Improve doxygen comment. Improve layout, add trailing '.' in function description, capitalize first letter of description, fix typo, turn \0 into \\0. Move the detailed description after @defgroup so that it is taken into account.

fix Doxygen warnings/comments

make varbuf functions treat AP_VARBUF_UNKNOWN consistently, improve docs ap_varbuf_pdup(): copying the whole buffer in case strlen == AP_VARBUF_UNKNOWN does not make sense as the caller can not set the exact buffer size, only a minimum. No API change as previously the behavior with AP_VARBUF_UNKNOWN was undocumented. regsub_core(): Checking for vb->buf is useless, it cannot be NULL unless ap_varbuf_init has not been called. ap_varbuf_cfg_getline(): Initially, allocate enough memory to hold an empty line. If strlen == AP_VARBUF_UNKNOWN, use strlen(buf) instead of undefined behavior.

Clean up size_t abuse, part 2. ap_malloc/calloc/realloc are explicitly excluded from this cleanup as they must be signature identical to the clib functions, and although the definition of size_t has been flakey, the definition of those functions appears to be generally clean since ANSI C.

Limit length of lines in .htaccess to 8K again, to reduce DoS potential. Make ap_varbuf_cfg_getline() strictly enforce the max_len parameter.

* include/util_varbuf.h: Doxygen tweaks.

Improve handling of maxlen = APR_SIZE_MAX, noticed by Jim. Use apr_pregsub_ex() and maxlen = 0 for unlimited in mod_substitute.

Limit ap_pregsub() to 64K, add ap_pregsub_ex() for longer strings and with better error reporting. Modify ap_varbuf_regsub() to be similar to ap_pregsub_ex().

Some varbuf enhancements: - Introduce new ap_varbuf_pdup() and ap_varbuf_regsub() functions. - Fix some bugs in ap_varbuf_strmemcat(). - Make ap_varbuf.buf point to an empty string if no buffer has been allocated, yet.

Actually commit the header added in r1157354