Merge branch 'dd/retire-efgrep'

Prepare for GNU [ef]grep that throw warning of their uses.

* dd/retire-efgrep:
  t: convert fgrep usage to "grep -F"
  t: convert egrep usage to "grep -E"
  t: remove \{m,n\} from BRE grep usage
  CodingGuidelines: allow grep -E