* src/augtool.c: make second argument for 'set' optional
* src/augtool.c: make second argument for 'set' optional

This makes it possible to set a tree node's value to the empty string
with
  set /foo ""
and
  set /foo

jtopjian/augeas,MikaelSmith/augeas,hercules-team/augeas-do-not-use,dafugg/augeas,pevalme/augeas,kumy/augeas,jasperla/augeas,lutter/augeas,mchf/augeas,lutter/augeas,jtopjian/augeas,hercules-team/augeas-do-not-use,lutter/augeas,kumy/augeas,kumy/augeas,ptoscano/augeas,mlichvar/augeas,manandbytes/augeas,bkearney/augeas,kunkku/augeas,jjlin/augeas,hercules-team/augeas,bkearney/augeas,jasperla/augeas,bkearney/augeas,kunkku/augeas,kumy/augeas,mlichvar/augeas,jjlin/augeas,pevalme/augeas,GeoffWilliams/augeas,GeoffWilliams/augeas,domcleal/augeas,kunkku/augeas,dafugg/augeas,ptoscano/augeas,dafugg/augeas,domcleal/augeas,hercules-team/augeas,pevalme/augeas,mchf/augeas,camptocamp/augeas,MikaelSmith/augeas,domcleal/augeas,raphink/augeas,hercules-team/augeas-do-not-use,bkearney/augeas,camptocamp/augeas,camptocamp/augeas,mlichvar/augeas,jjlin/augeas,camptocamp/augeas,manandbytes/augeas,ptoscano/augeas,raphink/augeas