*   Match definition of c_char in os/raw.rs with the libc definition

    Due to historic reasons, os/raw.rs redefines types for c_char from
    libc, but these didn't match. Now they do :).

*   Enable signal reset on exec for L4Re

    L4Re has full signal emulation and hence it needs to reset the
    signal set of the child with sigemptyset. However, gid and uid
    should *not* be set.

This commit adds the needed modifications to compile the std crate
for the L4 Runtime environment (L4Re).

A target for the L4Re was introduced in commit:
c151220

In many aspects implementations for linux also apply for the L4Re
microkernel.

Two uncommon characteristics had to be resolved:
* L4Re has no network funktionality
* L4Re has a maximum stacksize of 1Mb for threads

Co-authored-by: Sebastian Humenda <[email protected]>

As suggested in the discussion of PR rust-lang#43972, std should provide a uniform API to
all platforms. Since there's no networking on L4Re, this now is a module in
`sys::net` providing types and functions/methods returning an error for each
action.

The default min stack size value is smaller on l4re and therefore
this value has to be different depending on the platform.