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.
Merge branch 'master' of https://github.com/libimobiledevice/libimobiledevice