[PARSER] Add FAKEEOFMARK for expandstr

[PARSER] Add FAKEEOFMARK for expandstr

Previously expandstr used the string "" to indicate that it needs to be
treated just like a here-doc except that there is no terminator.  However,
the string "" is in fact a valid here-doc terminator so now that we deal
with it correctly expandstr no longer works in the presence of new-lines
in the prompt.

This patch introduces the FAKEEOFMARK macro which does not equal any
real EOF marker but is distinct from the NULL pointer which is used to
indicate non-here-doc contexts.

Thanks to Markus Triska for reporting this regression.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>