mail: Don't trap signals we shouldn't.

When in interactive mode, trap SIGINT, SIGHUP, and tty-related signals.
Otherwise, leave signals untouched as required by POSIX.

MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D50011

(cherry picked from commit 59597032c948586257f123f57bbcfbad02a2cd1b)

mail: Exit non-zero on failure to collect mail.

While here, avoid non-portable pipe2() in the regression test.

Fixes: 59597032c948
MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D50069

(cherry picked from commit 09bc6a5d5b3333bc91e5f9fdf1e7bb282c4aea5a)

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

(cherry picked from commit 42fd47ef81138ae53ba77add5a400f67af8e6263)