sigaction() problems

sigaction() problems

Working out an example of non-restarting syscalls I found three
problems.

  - sigaction warns if there are no flags in the SigAction structure.  I
    think the SigAction constructor should treat the SigSet and flags
    args as optional.  Minimalist patch appended.

	$ ./perl -MPOSIX=:signal_h -we '
	    sigaction 2, new POSIX::SigAction sub { }'
	Use of uninitialized value at -e line 1.
	$ ./perl -MPOSIX=:signal_h -we '
	    sigaction 2, new POSIX::SigAction sub { }, undef, 0'
	$ _

  - POSIX::constant warns on an arg-less macro.

	$ ./perl -MPOSIX=:signal_h -lwe 'print SIGALRM'
	Use of uninitialized value at /usr/local/lib/perl5/POSIX.pm line 197.
	14
	$ ./perl -MPOSIX=:signal_h -lwe 'print SIGALRM(0)'
	14
	$ _

  - sigaction doesn't actually work.

	$ ./perl -MPOSIX=:signal_h -we '
	    sigaction SIGALRM(0),
			new POSIX::SigAction sub { die "alarm\n" }, undef, 0;
	    kill "ALRM", $$'
	SIGALRM handler "CODE(0x223970)" not defined.
	$ _

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