getopt_long returns an int, not a char

On platforms with an unsigned char, such as Arm, this results in
always taking error paths around initialization.

Fixes https://github.com/fangfufu/httpdirfs/issues/103

src/main.c

@@ -169,7 +169,7 @@ void parse_config_file(char ***argv, int *argc)
 static int
 parse_arg_list(int argc, char **argv, char ***fuse_argv, int *fuse_argc)
 {
-    char c;
+    int c;
     int long_index = 0;
     const char *short_opts = "o:hVdfsp:u:P:";
     const struct option long_opts[] = {