Copy stable/10@r272459 to releng/10.1 as part of
the 10.1-RELEASE process.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Add a comment describing why it's important for the values in this
file to be correct, and how to generate them automatically.

Caused much pain and suffering for: peter

Replace our ancient dtoa/strtod implementation with the gdtoa
package, a more recent, generalized set of routines. Among the
changes:
- Declare strtof() and strtold() in stdlib.h.
- Add glue to libc to support these routines for all kinds
of ``long double''.
- Update printf() to reflect the fact that dtoa works slightly
differently now.

As soon as I see that nothing has blown up, I will kill
src/lib/libc/stdlib/strtod.c. Soon printf() will be able
to use the new routines to output long doubles without loss
of precision, but numerous bugs in the existing code must
be addressed first.

Reviewed by: bde (briefly), mike (mentor), obrien