Cygwin and Mingw floating-point fixes.

Deal with silent-underflow errors in float4 for cygwin and mingw by
using our strtof() wrapper; deal with misrounding errors by adding
them to the resultmap. Some slight reorganization of declarations was
done to avoid duplicating material between cygwin.h and win32_port.h.

While here, remove from the resultmap all references to
float8-small-is-zero; inspection of cygwin output suggests it's no
longer required there, and the freebsd/netbsd/openbsd entries should
no longer be necessary (these date back to c. 2000). This commit
doesn't remove the file itself nor the documentation references for
it; that will happen in a subsequent commit if all goes well.

src/include/port.h

@@ -385,6 +385,11 @@ extern int	isinf(double x);
 extern float strtof(const char *nptr, char **endptr);
 #endif
 
+#ifdef HAVE_BUGGY_STRTOF
+extern float pg_strtof(const char *nptr, char **endptr);
+#define strtof(a,b) (pg_strtof((a),(b)))
+#endif
+
 #ifndef HAVE_MKDTEMP
 extern char *mkdtemp(char *path);
 #endif