[CFT] rewrite of the merge(1) utility

Hi all,

I was botherd to not have the merge(1) utility available in base (for etcupdate)
when building base WITHOUT_RCS.

So I have rewritten a merge(1) utility which should be compatible.

I used the 3-way merge code from the fossil VCS instead of making it call diff3.
All I have done from the fossil code is adapting it to use sbuf(9).

The bonus for end users is the merge from fossil can resolve situation where the
diff3 in base cannot. (which explains a "failure" with the GNU RCS test suite)

meaning etcupdate will be more happy merge configuration files.

Another bonus is merge -e is now working again (it is broken in base from my
testings).

It should be able to replace entirely GNU RCS merge(1). From my testing only one
output with co(1) is slightly different but harmless.

I would appreciate more testing in particular from rcs(1).

(No I have no intention to rewrite ci(1), co(1) or rcs*)

Best regards,
Bapt