Rationalize grep behavior

[dspinellis]

• By default list only matching elements.
• If exactly one -L -l or -v -o -c etc is specified, output only what is specified.
• If --matching-lines is specified in addition to an option in the above list, also output the corresponding matching lines.
• If more than one of the -L -l or -v -o -c etc options is specified generate one output per option.
• If more than one -e option is specified, output one stream per option (do not implement this if internally the REs are merged into one).

[dspinellis]

Also grep behaves strangely when at the end of a pipeline.

$ dgsh -c 'cat /etc/motd | grep WARRANTY'

The programs included with the Debian GNU/Linux system are free software;
the exact distribution terms for each program are described in the
individual files in /usr/share/doc/*/copyright.

Debian GNU/Linux comes with ABSOLUTELY NO WARRANTY, to the extent

[mfragkoulis]

Commit 8a3c648 points to updated grep submodule HEAD.
The output streams work fine when they are used independently.
The implemented examples within /examples work fine.

TODO: Combining file streams (-l, -L) with inverted matches stucks in an fwrite() call.

[dspinellis]

Buffering may be needed here. We should not implement buffering at each command. Passing the grep output through a no-op dgsh-tee should solve this.

[mfragkoulis]

The wiki is updated.

[mfragkoulis]

grep -c '@[ ]*define[ ][ ]*[a-zA-Z_][a-zA-Z0-9_]*(' produces a false match in test-dgsh.sh.

The pattern tries to identify macro definitions.

The baseline is given by the equivalent sgsh example (sgsh is dgsh's predecessor).

The test succeeds on MacOS, but fails on Linux.