pgsql-server: Split out everything that looks like a function call from

Log Message:
-----------
Split out everything that looks like a function call from c_expr into
a separate production func_expr.  This allows us to accept all these
variants in the backwards-compatible syntax for creating a functional
index; which beats documenting exactly which things work and which don't.
Interestingly, it also seems to make the generated state machine a little
bit smaller.

Modified Files:
--------------
    pgsql-server/src/backend/parser:
        gram.y (r2.475 -> r2.476)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/parser/gram.y.diff?r1=2.475&r2=2.476)