Match c_char definitions and enable signal reset for L4Re

*   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.

src/libstd/sys/unix/mod.rs

@@ -44,6 +44,7 @@ pub mod fd;
 pub mod fs;
 pub mod memchr;
 pub mod mutex;
+#[cfg(not(target_os = "l4re"))]
 pub mod net;
 pub mod os;
 pub mod os_str;