[foreign] RFR 8212061: Fix jextract command line errors and crashes

[Maurizio Cimadamore]

This patch fixes two further problems with the jextract tool:

1) jextract will attempt to generate a jarfile where the extracted 
header is - if that is a system header, this leads to permission issues 
(e.g. cannot write to xyz)
2) If some option is used but no actual header file is specified, 
jextract crashes with exception


(1) has been addressed by using working dir, which should lead to more 
predictable behavior. (2) has been fixed by checking for absence of 
input files, and issuing an error if that happens.

Webrev:

http://cr.openjdk.java.net/~mcimadamore/panama/8212061/

Cheers
Maurizio