fgrep uses different matcher from grep, but there is no reason why it
must be so.  Further more, fgrep matcher is slower than grep matcher in
many cases, because it never uses DFA.  This patch converts fgrep to grep
any time.

By the way, if `FGREP_NO_DFA' env. is set, fgrep still uses fgrep matcher.
It may be required in some cases.  fgrep matcher requires less memory
than grep matcher even if a pattern is too long, and/or is faster than
it in some cases to have multiple patterns.