Add wcstof(3) and wcstold(3).

ok miod@

Fix conversion of numbers without an exponent.

ok martynas@

Add a few missing functions so that wctype.h/wchar.h are more or less
uptodate, namely, wcsto(u)l(l) family, wcstod, stubs from wctrans/towctrans

crank minor.

okay millert@, jmc@.