Keep rejects from failed patch hunks

As is, customizepkg will exit with an error if the patch doesn't
apply cleanly (which indicates a "merge conflict" between changes
made by the package maintainer and local customizations).
This change checks patch's exit code and only exits when there is
a problem more serious than failing patch hunks.
Also clean up the .rej files unless MODIFY option is given.
Preview rejects
Merge branch 'ackalker-keep-rejects'

* Keep reject files
* Preview them