mail: Further refine the SIGINT test.

• Wait at least 1-2 s before sending the second SIGINT.
• If the child is still running after 15 s, send a SIGKILL.
• Improve the exit status checks.

Fixes: 59597032c948
MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D50092