Revert my previous change to "-S/--skip" in favour of the two options

Revert my previous change to "-S/--skip" in favour of the two options
"-E/--skip-stderr" and "-S/--skip-stdout".  Both of them support
omitting the number of lines to skip, in which case all output on the
respective file descriptor is skipped.  "--skip" is kept as an alias for
"--skip-stdout" for backwards compatibility with recent releases.

Also, print a message if no (non-skipped) stdout/stderr output is
available.  This fixes a segfault if the remote command prints no
output.

git-svn-id: https://nagiosplug.svn.sourceforge.net/svnroot/nagiosplug/nagiosplug/trunk@1692 f882894a-f735-0410-b71e-b25c423dba1c