Core: added NGX_REGEX_MULTILINE for 3rd party modules. Notably, NAXSI is known to misuse ngx_regex_compile() with rc.options set to PCRE_CASELESS | PCRE_MULTILINE. With PCRE2 support, and notably binary compatibility changes, it is no longer possible to set PCRE[2]_MULTILINE option without using proper interface. To facilitate correct usage, this change adds the NGX_REGEX_MULTILINE option.