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.

src/core/ngx_regex.c

diff --git a/src/core/ngx_regex.c b/src/core/ngx_regex.c
index 991728b..bebf3b6 100644
--- a/src/core/ngx_regex.c
+++ b/src/core/ngx_regex.c
@@ -159,7 +159,11 @@
         options |= PCRE2_CASELESS;
     }
 
-    if (rc->options & ~NGX_REGEX_CASELESS) {
+    if (rc->options & NGX_REGEX_MULTILINE) {
+        options |= PCRE2_MULTILINE;
+    }
+
+    if (rc->options & ~(NGX_REGEX_CASELESS|NGX_REGEX_MULTILINE)) {
         rc->err.len = ngx_snprintf(rc->err.data, rc->err.len,
                             "regex \"%V\" compilation failed: invalid options",
                             &rc->pattern)
@@ -275,7 +279,11 @@
         options |= PCRE_CASELESS;
     }
 
-    if (rc->options & ~NGX_REGEX_CASELESS) {
+    if (rc->options & NGX_REGEX_MULTILINE) {
+        options |= PCRE_MULTILINE;
+    }
+
+    if (rc->options & ~(NGX_REGEX_CASELESS|NGX_REGEX_MULTILINE)) {
         rc->err.len = ngx_snprintf(rc->err.data, rc->err.len,
                             "regex \"%V\" compilation failed: invalid options",
                             &rc->pattern)

src/core/ngx_regex.h

diff --git a/src/core/ngx_regex.h b/src/core/ngx_regex.h
index 74e694d..182373a 100644
--- a/src/core/ngx_regex.h
+++ b/src/core/ngx_regex.h
@@ -37,6 +37,7 @@
 
 
 #define NGX_REGEX_CASELESS     0x00000001
+#define NGX_REGEX_MULTILINE    0x00000002
 
 
 typedef struct {