parser: Add syntax stack for recursive parsing

Without a stack of syntaxes we cannot correctly these two cases
together:

        "${a#'$$'}"
        "${a#"${b-'$$'}"}"

A recursive parser also helps in some other corner cases such
as nested arithmetic expansion with paratheses.

This patch adds a syntax stack allocated from the stack using
alloca.  As a side-effect this allows us to remove the naked
backslashes for patterns within double-quotes, which means that
EXP_QPAT also has to go.

This patch also fixes removes any backslashes that precede right
braces when they are present within a parameter expansion context,
and backslashes that precede double quotes within inner double
quotes inside a parameter expansion in a here-document context.

The idea of a recursive parser is based on a patch by Harald van
Dijk.

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

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