kpatch-build: getopts arg parsing cleanup

Cleanup the kpatch-build argument parsing a little bit:
- gracefully handle no args
- allow white space in filenames
- use 'eval set -- $options' to allow use of $1 and $2 variables
kpatch-build: add "no patch file specified" error
Merge pull request #100 from jpoimboe/kpatch-build-noargs

kpatch-build: getopts arg parsing cleanup