handle backslash and ampersand correctly; from Diomidis; examples:

handle backslash and ampersand correctly; from Diomidis; examples:
echo '\foo' | sed -e 's/\\/\\e/' -e 's/^/\\\&/'
echo "foo" | sed -e 's/./\\\&/' produces \foo instead of \&oo.
echo "foo" | sed -e 's/./\\&/' produces \&oo instead of \foo.

SCCS-vsn: usr.bin/sed/compile.c 5.6