asa: Error out if writing to stdout failed.

MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D40562

(cherry picked from commit 9e379f9639d51442ad1d5e1800c687ef2c954901)

asa: Read from stdin if *argv is "-".

MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D40563

(cherry picked from commit f08f90e6987775f88d25efbd8762c361819f40ba)

asa: Add some unit tests.

MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D40564

(cherry picked from commit b7412da2e122d9274341ef840c6918409bc523b3)