ap_expr: open string expressions to the <word>.

Introduces the syntax "%{:<word>:}", borrowed from the <var>'s one, and which
likewise can be embedded anywhere in a string expression (the same reserved
character ':' gets reused in an unambiguous manner).

This allows the two types of expressions (boolean and string) to now share
fully the same language set, namely: strings, lists, vars, regexes, backrefs,
functions with multiple or complex arguments, and especially combinations
thereof.

Most of them were reserved to boolean expressions only, while complex string
constructions can also benefit to, well, strings. The <word> construct allows
that (say the syntax "%{:<word>:}" looks like a temporary variable constructed
in a string).

Since string expressions may now have to deal with lists (arrays), they also
need a way to produce/extract strings from list and vice versa. This can be
done with the new "join" and "split" operators, while the new substitution
regexes (like "s/<pattern>/<substitute>/<flags>") may be used to manipulate
strings in place. All this of course available for both string and boolean
expressions.

Tests and doc updates upcoming..



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1810605 13f79535-47bb-0310-9956-ffa450edef68

server/util_pcre.c

@@ -246,6 +246,10 @@ AP_DECLARE(int) ap_regexec_len(const ap_regex_t *preg, const char *buff,
         options |= PCREn(NOTBOL);
     if ((eflags & AP_REG_NOTEOL) != 0)
         options |= PCREn(NOTEOL);
+    if ((eflags & AP_REG_NOTEMPTY) != 0)
+        options |= PCREn(NOTEMPTY);
+    if ((eflags & AP_REG_ANCHORED) != 0)
+        options |= PCREn(ANCHORED);
 
 #ifdef HAVE_PCRE2
     /* TODO: create a generic TLS matchdata buffer of some nmatch limit,