[BUILTIN] Allow return in loop conditional to set exit status

[BUILTIN] Allow return in loop conditional to set exit status

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=332954

When return is used in a loop conditional the exit status will
be lost because we always set the exit status at the end of the
loop to that of the last command executed in the body.

This is counterintuitive and contrary to what most other shells do.

This patch fixes this by always preserving the exit status of
return when it is used in a loop conditional.

The patch was originally written by Gerrit Pape <pape@smarden.org>.

Reported-by: Stephane Chazelas <stephane_chazelas@yahoo.fr>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>