Fix building warnings on Cygwin

src/acl.c

@@ -343,7 +343,7 @@ trimwhitespace(char *str)
     char *end;
 
     // Trim leading space
-    while (isspace(*str))
+    while (isspace((unsigned char)*str))
         str++;
 
     if (*str == 0)   // All spaces?
@@ -351,7 +351,7 @@ trimwhitespace(char *str)
 
     // Trim trailing space
     end = str + strlen(str) - 1;
-    while (end > str && isspace(*end))
+    while (end > str && isspace((unsigned char)*end))
         end--;
 
     // Write new null terminator

src/http.c

@@ -103,7 +103,7 @@ get_header(const char *header, const char *data, int data_len, char **value)
     while ((len = next_header(&data, &data_len)) != 0)
         if (len > header_len && strncasecmp(header, data, header_len) == 0) {
             /* Eat leading whitespace */
-            while (header_len < len && isblank(data[header_len]))
+            while (header_len < len && isblank((unsigned char)data[header_len]))
                 header_len++;
 
             *value = malloc(len - header_len + 1);

src/utils.c

@@ -92,7 +92,7 @@ int
 ss_isnumeric(const char *s) {
     if (!s || !*s)
         return 0;
-    while (isdigit(*s))
+    while (isdigit((unsigned char)*s))
         ++s;
     return *s == '\0';
 }
@@ -140,10 +140,12 @@ run_as(const char *user)
                     return 0;
                 }
 
+#ifndef __CYGWIN__
                 if (initgroups(pwd->pw_name, pwd->pw_gid) == -1) {
                     LOGE("Could not change supplementary groups for user '%s'.", pwd->pw_name);
                     return 0;
                 }
+#endif
 
                 if (setuid(pwd->pw_uid) != 0) {
                     LOGE(