fnmatch: pattern syntax can no longer fail

Whenever an expected pattern syntax rule cannot be matched, the
character starting the rule loses its special meaning and the parsing
is resumed:
- backslash at the end of pattern string matches itself.
- Error in [:keyword:] results in set containing :\[dekorwy.

Unit test 1307 updated for this new situation.

Closes #2273

tests/unit/unit1307.c

@@ -25,7 +25,6 @@
 
 #define MATCH   CURL_FNMATCH_MATCH
 #define NOMATCH CURL_FNMATCH_NOMATCH
-#define RE_ERR  CURL_FNMATCH_FAIL
 
 struct testcase {
   const char *pattern;
@@ -135,6 +134,8 @@ static const struct testcase tests[] = {
   { "[^[:blank:]]",             "\t",                     NOMATCH },
   { "[^[:print:]]",             "\10",                    MATCH },
   { "[[:lower:]][[:lower:]]",   "ll",                     MATCH },
+  { "[[:foo:]]",                "bar",                    NOMATCH },
+  { "[[:foo:]]",                "f]",                     MATCH },
 
   { "Curl[[:blank:]];-)",       "Curl ;-)",               MATCH },
   { "*[[:blank:]]*",            " ",                      MATCH },
@@ -172,7 +173,7 @@ static const struct testcase tests[] = {
   { "x",                        "",                       NOMATCH },
 
   /* backslash */
-  { "\\",                       "\\",                     RE_ERR },
+  { "\\",                       "\\",                     MATCH },
   { "\\\\",                     "\\",                     MATCH },
   { "\\\\",                     "\\\\",                   NOMATCH },
   { "\\?",                      "?",                      MATCH },