The NumericOnly grammar production accepted ICONST, + ICONST, - ICONST,
FCONST, and - FCONST, but for some reason not + FCONST.  This led to
strange inconsistencies like

regression=# set random_page_cost = +4;
SET
regression=# set random_page_cost = 4000000000;
SET
regression=# set random_page_cost = +4000000000;
ERROR:  syntax error at or near "4000000000"

(because 4000000000 is too large to be an ICONST).  While there's
no actual functional reason to need to write a "+", if we allow
it for integers it seems like we should allow it for numerics too.

It's been like that forever, so back-patch to all supported branches.

Discussion: https://postgr.es/m/30908.1496006184@sss.pgh.pa.us