update_unit_activity() fixes:
* Do not use clears silently the orders of the units ;
* Really stop units doing illegal activities ;
* Do not check for adjacent units if no activity has been completed.

From a report by Jacob Nevins (jtn@gna)

See gna bug #22136

git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@25195 a0f10bec-cc02-0410-94fc-a9cfff90b4cd
Make unit activity updating more predicable:
* in first phase, update activity (road|irrigation|mine building, terraform,
pillage, pollution cleaing etc.) ;
* in second phase, execute unit orders ;
* in third phase, remember the unit activity and send unit info.

See gna patch #4807

git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@25197 a0f10bec-cc02-0410-94fc-a9cfff90b4cd