[tschwinge/sendmmsg] Add recvmmsg and sendmmsg to the generic glibc API

[Roland McGrath]

Don't put __sendmmsg into the public ABI (i.e. GLIBC_2.17 set).  If it's
used by other shared libraries and those uses are in functions that exist
in some standard-specified or traditional name space, then put it in
GLIBC_PRIVATE (no .abilist files are touched for this).  If the only uses
from other shared libraries are in GNU extension functions, then they can
just use plain sendmmsg and we don't need __sendmmsg at all.

I don't see any callers of recvmmsg at all, so it certainly doesn't need
the __recvmmsg name.

All files using stub_warning need #include <stub-tag.h>.

For every new file you add, make the top line a descriptive comment,
copyright notice on the second line.

When posting patches, always use the canonical form with the log entries
outside the patch and not in patch form.


Thanks,
Roland