Move fork implementation to redox-exec

See merge request !345

strtof, strtod: Fix parsing floats with exponents

See merge request !344

Userspace fexec

See merge request redox-os/relibc!343

With this change, gcc can now successfully compile a tiny program that
printfs an integer returned from a function, from a dynamically linked
library that it compiled as well.

Rustc however, is orders of magnitude more complex, and the next step is
to fix constructors which require access to `environ`, in ld.so

The #[global_allocator] used by relibc is only relibc's business.
Symbols should therefore not leak if another applications for example
wants to override the system allocator (and for bootstrap, which links
to relibc but uses its own #[global_allocator] based on libc functions).

Rust's libstd for example uses environ when using the `envs` builder
method for spawning processes, and therefore relibc cannot simply assume
environ will always point to the internal relibc env var Vec.