pgsql: Optionally don't error out due to preexisting slots in commandli

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

Branch
------
REL9_5_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/0e8e48b0da6ea00f3dbcb659542b0c81a97d1253

Modified Files
--------------
doc/src/sgml/ref/pg_receivexlog.sgml   |   10 ++++++++
doc/src/sgml/ref/pg_recvlogical.sgml   |   10 ++++++++
src/bin/pg_basebackup/pg_receivexlog.c |   11 ++++++--
src/bin/pg_basebackup/pg_recvlogical.c |    9 ++++++-
src/bin/pg_basebackup/streamutil.c     |   44 ++++++++++++++------------------
src/bin/pg_basebackup/streamutil.h     |    4 +--
6 files changed, 58 insertions(+), 30 deletions(-)