[FIXED JENKINS-25358] Remove extra fields from getFieldOrder()

Remove fields which are not part of struct _SHELLEXECUTEINFO.
Use a jna Union type for the DUMMYUNIONNAME union structure

JNAerator ( https://code.google.com/p/jnaerator/ ) was helpful
for generating the Java code from the C struct.
Merge branch 'JENKINS-25358' of github.com:rodrigc/jenkins
[JENKINS-25358] Noting merge of #1445.