MFC r361884: sed: attempt to learn about hex escapes (e.g. \x27)

MFC r361884: sed: attempt to learn about hex escapes (e.g. \x27)

Somewhat predictably, software often wants to use \x27/\x24 among others so
that they can decline worrying about ugly escaping, if said escaping is even
possible. Right now, this software is using these and getting the wrong
results, as we'll interpret those as x27 and x24 respectively. Some examples
of this, when an exp-run was ran, were science/octopus and misc/vifm.

Go ahead and process these at all times.  We allow either one or two digits,
and the tests account for both.  If extra digits are specified, e.g. \x2727,
then the third and fourth digits are interpreted literally as one might
expect.