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

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

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

--
zfs_main.c: In function 'zfs_do_diff':
zfs_main.c:7178:9: error: implicit declaration of function 'sigignore' [-Werror=implicit-function-declaration]
  (void) sigignore(SIGPIPE);
         ^~~~~~~~~

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