sed: rewrite the main loop.

Rewrite the main loop of the "sed s/..." command, shortening it by ten
lines and simplifying it by removing the switch statement implementing
/g, /1, and /2 separately and repetitively.

This will be needed to bring a fix from OpenBSD later.

Obtained from: OpenBSD (schwarze CVS Rev. 1.18)
MFC after: 3 weeks