Fix C++ compilation warnings/errors under g++ 8.2.

Fix C++ compilation warnings/errors under g++ 8.2.

In metadata.cc:
    warning: In the GNU C Library, "major" is defined by
    <sys/sysmacros.h>.  For historical compatibility, it is currently
    defined by <sys/types.h> as well, but we plan to remove this soon.
    To use "major", include <sys/sysmacros.h> directly. If you did not
    intend to use a system-defined macro "major", you should undefine it
    after including <sys/types.h>.

In third_party/sha1.cc:
    error: types may not be defined within __builtin_offsetof

Alter the code to always assume that buffers are
unaligned, which should be correct at the cost of performance.  This
shouldn't matter much in practice since SHA-1 is mostly not being used
by Cumulus any longer.

In third_party/sha256.cc:
    warning: #warning "_BSD_SOURCE and _SVID_SOURCE are deprecated, use
    _DEFAULT_SOURCE"