feat(grep): treat - operand as standard input (#331)

`grep PATTERN -` failed with "grep: -: No such file or directory" instead
of reading stdin. GNU treats `-` as a FILE operand naming standard input,
labelled `(standard input)` wherever a file name would appear.

- `-` reads stdin and is labelled `(standard input)` in the multi-file
  prefix and in -l/-L/-c output; a lone `-` gets no prefix, and -h
  suppresses it like any other file name.
- stdin is a stream: repeated `-` operands see it drained by the first,
  so `grep -c pat - -` prints `(standard input):N` then
  `(standard input):0`.
- `-` bypasses glob expansion, -r recursion and --include/--exclude,
  which all filter on a file name stdin does not have. Under -r the
  file-name prefix is now forced only when a real path is searched, so
  `grep -r pat -` prints bare lines like GNU.
- Empty stdin is a valid empty stream (exit 1, `-c` prints 0); absent
  stdin reads as empty, matching `cat -`.

Unskips 4 busybox spec tests and corrects the reason on 2 that stay
skipped for unrelated causes (spec-runner path rewriting, missing -s).

All expectations verified against GNU grep 3.12; comparison fixtures are
recorded from it and locked.

Signed-off-by: Lars Trieloff <lars@trieloff.net>