MFC 362017, 362039, 362071:

Read commands from stdin when -f - is passed to sed(1)

This patch teaches sed to interpret a "-" in a special way when given
as an argument to the -f flag.

This behavior is also present in GNU sed.

PR: 244872
Tested by: antoine (exp-run)
Reviewed by: pfg, tobik (older version)
Approved by: pfg (src)
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D24079

Remove duplicate lines from sed tests

Reported by: yuripv
Approved by: pfg (src)

Remove some more duplicate test cases I accidentally committed

Reported by: markj, yuripv