Change edit/gedit/setpid/gsetpid to return 0/-1 to indicate success or failure

Change edit/gedit/setpid/gsetpid to return 0/-1 to indicate success or failure
as all other helpers do.

Makes it clear that X* functions are in charge of deciding which CMD_* value
they return to main edting loop. Reduces unnecessary copying/restoring of
original partition information.

No intentional functional change.