GXemul Code2

Adding stdio.h include, required for build with some gcc versions on Linux. Noticed by Michael Mikonos.

Minihack to allow reading the bootloader from IRIX formatted disk, NetBSD install CDROM, and OpenBSD install CDROM.

SGI boot / partition work-in-progress.

Some minor refactoring of arcbios environment variables etc, to attempt to get emulated O2 boot-from-disk image (or cdrom image) to work better.

The ECOFF file loader can now (barely) load relocated binaries, such as "sash" from an IRIX installation on my real O2.

One -v on the command line should be enough to see file loading details.

Enabling -Werror again; seems to work on my machine for clang and most GNU C++ versions (up to 9).

bootblock.cc now parses SGI disk boot blocks, and tries to load the OSLoader (sash), if booting without an explicitly provided OS kernel.

Slightly nicer indentation for ARC/ARCS boot info.

Reshuffling the TODO.