killall: add -q flag to suppress error message when no processes are matched

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PR:		bin/30542
Submitted by:	Tony Finch <dot@dotat.at> (original version)
MFC after:	1 week
libc: Access _sigintr more efficiently.

The variable _sigintr is not exported via the version script; therefore,
tell the compiler that no indirection (to allow interposition) is needed.
When decoding SLEB128, make sure sign extension is performed for
64-bit integers.

MFC after:	3 days