Optionally don't error out due to preexisting slots in commandline utilities.

 Optionally don't error out due to preexisting slots in commandline utilities.
 
 pg_receivexlog and pg_recvlogical error out when --create-slot is
 specified and a slot with the same name already exists. In some cases,
 especially with pg_receivexlog, that's rather annoying and requires
 additional scripting.
 
 Backpatch to 9.5 as slot control functions have newly been added to
 pg_receivexlog, and there doesn't seem much point leaving it in a less
 useful state.
 
 Discussion: 20150619144755.GG29350@alap3.anarazel.de