COMMON: For portability, use generic_string() instead of string()

Thanks to darkstar.
COMMON: Replace MKID_BE by MKTAG

MKID_BE relied on unspecified behavior of the C++ compiler,
and as such was always a bit unsafe. The new MKTAG macro
is slightly less elegant, but does no longer depend on the
behavior of the compiler.
Inspired by FFmpeg via ScummVM.