Handle return values from fchown

The function fchown is annotated with warn_unused_result.  As
formerly used in the code, it would emit a compiler warning
```warning: ignoring return value of ‘fchown’, declared with
attribute warn_unused_result [-Wunused-result]```

This commit notes the return value and emits appropriate error/logging
messages if the call fails, creating more traceable results and
satisfying the compiler.

src/env_universal_common.cpp

@@ -866,7 +866,8 @@ bool env_universal_t::sync(callback_data_list_t *callbacks)
         struct stat sbuf;
         if (wstat(vars_path, &sbuf) >= 0)
         {
-            fchown(private_fd, sbuf.st_uid, sbuf.st_gid);
+            if (0 > fchown(private_fd, sbuf.st_uid, sbuf.st_gid))
+                UNIVERSAL_LOG("fchown() failed");
             fchmod(private_fd, sbuf.st_mode);
         }
 

src/history.cpp

@@ -1453,7 +1453,10 @@ bool history_t::save_internal_via_rewrite()
                 if (wstat(new_name, &sbuf) >= 0)
                 {
                    /* Success */
-                    fchown(out_fd, sbuf.st_uid, sbuf.st_gid);
+                    if (0 > fchown(out_fd, sbuf.st_uid, sbuf.st_gid))
+                    {
+                        debug(2, L"Error when changing ownership of history file");
+                    }
                     fchmod(out_fd, sbuf.st_mode);
                 }