Remove preamble from BUGS section as suggested by @markj

Remove the "-e flags containing quotes" test, it only fails because
of the multiple -e flag issue. Add references to gzip(1) and zstd(1)
to the man page and also document the two known problems.

The thought occurs that this is an opportunity to make minimal changes to the modest man page; @matthew remarked that it doesn't reference zstd(1) and I see gzip(1) is also missing. In addition it would probably make sense to add a BUGS section and document the known problems (regressions).

Change atf_skip to atf_expect_fail as suggested by @kevans.

In D25613#567335, @kevans wrote:

atf_expect_fail is better here, then if it succeeds it fails and we know it's safe to remove it.

Next round of fixes

I'll wait for guidance before continuing.

Add a test for problems with -e PATTERN when it is quoted. Add a
test for problems with long flags (e.g. --ignore-case), particularly
when reading from stdin. Add a test for -e PATTERN followed by
other flags.