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
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D24079