Properly compensate mtype size differences running cross-chroot

This is the second attempt to make fakeroot work properly in the following scebario:

• The faked-* daemon is running on 'host' machine
• The libfakeroot-* library is preloaded inside chroot of same or different architecture
• The target architecture can be executed on 'host' machine

The first attempt introduced a regression (Bug #971928) so I identified the root cause of an issue and separated fake_msg structure from SysV IPC message buffer and implemented the magic-header search to compensate differing sizeof(struct fake_msg.mtype). The fact that modern fakeroot uses 64-bit fixed-width members except of mtype allows doing things like:

fakeroot env ARCH=x32 DIST=unstable pbuilder login

on amd64 host machine.

The current change has been tested on the following systems:

• amd64 - package tests pass, plain fakeroot call works, fakeroot env pbuilder also works for amd64, i386 and x32 base tarballs.
• i386 - package tests pass, plain fakeroot call works
• mips64el, mipsel - package tests pass, plain fakeroot call works, mipsel faked-sysv daemon correctly builds fakeroot package with libfakeroot-sysv.so preloaded inside mips64el chroot.
• s390x - package tests pass, plain fakeroot call works
• x32 - package tests pass, plain fakeroot call works