Update versions of rand crates for new rand core

See merge request !521

Make relibc-tests easy to run on Redox

See merge request redox-os/relibc!522

Remove ld.so's now unnecessary access() wrapper.

See merge request redox-os/relibc!520

Add sigaltstack test

See merge request redox-os/relibc!518

Impl net/if.h

See merge request !517

Remove the C-style "-1 or errno" e() function entirely

See merge request !516

Eliminate all errno-based error handling in the Pal trait

See merge request !515

Returning a negative number of bytes makes absolutely no sense, besides
the "-1 and errno" pattern, which is now converted to Result<_, Errno>.

Change a bunch of Pal functions to use Rusty error handling

See merge request !514

Preserve umask across execv.

See merge request redox-os/relibc!512

Move umask to a regular global variable.

See merge request !511

The umask value is per-process, so because all POSIX open-like functions
now occur via relibc, this is valid.

Implement realtime signals and remaining signal.h APIs

See merge request !493