idevicename: Remove leading newline from usage output
idevicename: Update usage output to match other tools and add debug option
docs: Update idevicename man page to match current usage
idevicename: Use lockdownd_get_device_name() helper
lockdown: Return error if getting a lockdown value failed to fix segfault

https://github.com/libimobiledevice/libimobiledevice/issues/67

The lockdownd_get_value() function is not properly handling an error
situation. The code logic always returned success as long as the lockdown
response was properly received. Thanks to Aaron Burghardt for spotting this.