Fix and change y command in the following ways:

Fix and change y command in the following ways:
- When 'n' is used as a delimiter escaping 'n' will remain a newline
  instead of becoming a 'n' character. This is how POSIX specifies how this
  should work. Other implementations tested also do this wrong.
- '[' and maybe other characters are not special during the parsing of the
  y command and don't need to be matched or treated special in any way.
- POSIX specifies that a backslash followed by anything other than the
  delimiter, 'n', and another backslash, as well as repeating characters in
  string1 are unspecified. Since the various implementations handle these
  situations in opposing ways choose to error out on them to prevent people
  falling into the pitfall of expecting identical behaviour on various
  implementations.

Inspired by the sed.1 patch by kshe59 <at> zoho <dot> eu
Feedback and OK millert@
Manpage bits OK jmc@