Add support for OR dependency operator

Add support for job dependencies jointed with OR operator (e.g.
"--depend=afterok:123?afternotok:124").
Start RELEASE_NOTES for v15.08
Remove redundant logic

There were several local strcmp functions that handled NULL input.
I removed the local functions and changed the code to use xstrcmp
from src/common/xstring.c instead. No changed in functionality.