--posix should disable the GNU regular expression escapes (\? \+ \| \w \s)

[sylvestre]

Found by the differential fuzzer (fuzz/fuzz_targets/fuzz_sed.rs).

With --posix, GNU sed drops the GNU regular expression escapes: \?, \+,
\| are not operators and \w, \s are not classes, they all stand for the
plain character. We keep honoring them.

$ printf 'a\nb\n0?\n' | sed --posix -e '/a\+/d'
a
b
0?
$ printf 'a\nb\n0?\n' | ./target/release/sed --posix -e '/a\+/d'
b
0?

Same for the others:

script	GNU --posix deletes	we delete
/0\?/d	0?	every line (empty match)
/a\+/d	nothing	a
/a|b/d	nothing	a and b
/\w/d	nothing	every line

Until this is fixed the fuzzer skips these scenarios; see KNOWN_ISSUES in
fuzz/fuzz_targets/fuzz_sed.rs.