core: add support of flags in regular expressions and highlight options, add irc options to customize/disable default nick highlight (task #11128)

New functions in C plugin API:
- string_regex_flags
- string_regcomp

New irc options:
- irc.look.highlight_server
- irc.look.highlight_channel
- irc.look.highlight_pv

Regex flags are supported in following options/commands:
- option weechat.look.highlight
- option weechat.look.highlight_regex
- options irc.look.highlight_{server|channel|pv}
- option relay.network.allowed_ips
- core command /filter
- irc command /list
- irc command /ignore
- rmodifier command /rmodifier

src/plugins/rmodifier/rmodifier-command.c

@@ -246,7 +246,8 @@ rmodifier_command_init ()
                              "     groups: action on groups found: comma separated "
                              "list of groups (from 1 to 9) with optional \"*\" "
                              "after number to hide group\n"
-                             "      regex: regular expression\n"
+                             "      regex: regular expression (case insensitive, "
+                             "can start by \"(?-i)\" to become case sensitive)\n"
                              "        del: delete a rmodifier\n"
                              "       -all: delete all rmodifiers\n"
                              "    default: restore default rmodifiers\n\n"

src/plugins/rmodifier/rmodifier.c

@@ -271,8 +271,8 @@ rmodifier_new (const char *name, const char *modifiers, const char *str_regex,
     if (!regex)
         return NULL;
 
-    if (regcomp (regex, str_regex,
-                 REG_EXTENDED | REG_ICASE) != 0)
+    if (weechat_string_regcomp (regex, str_regex,
+                                REG_EXTENDED | REG_ICASE) != 0)
     {
         weechat_printf (NULL,
                         _("%s%s: error compiling regular expression \"%s\""),