Use sigaction(2) instead of sigset(3) for portability

Use sigaction(2) instead of sigset(3) for portability

sigset(3) isn't portable.
This code fails to compile on platforms without sigset(3).
Use sigaction(2).

--
largest_file.c: In function 'main':
largest_file.c:75:9: error: implicit declaration of function 'sigset'; did you mean 'sigvec'? [-Werror=implicit-function-declaration]
  (void) sigset(SIGXFSZ, sigxfsz);
         ^~~~~~
         sigvec

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tomohiro Kusumi <kusumi.tomohiro@gmail.com>
Closes #8593