pgsql: Fix incorrect error message reported for non-existent users

Fix incorrect error message reported for non-existent users

Previously, lookups of non-existent user names could return "Success";
it will now return "User does not exist" by resetting errno.  This also
centralizes the user name lookup code in libpgport.

Report and analysis by Nicolas Marchildon;  patch by me

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/613c6d26bd42dd8c2dd0664315be9551475b8864

Modified Files
--------------
contrib/pg_upgrade/util.c    |   23 ++++--------
src/backend/libpq/auth.c     |   15 ++++----
src/backend/main/main.c      |   37 +------------------
src/bin/initdb/initdb.c      |   37 +++----------------
src/bin/psql/command.c       |    5 ++-
src/bin/psql/help.c          |   24 ++----------
src/bin/scripts/clusterdb.c  |    2 +-
src/bin/scripts/common.c     |   33 -----------------
src/bin/scripts/common.h     |    2 -
src/bin/scripts/createdb.c   |    2 +-
src/bin/scripts/createlang.c |    2 +-
src/bin/scripts/createuser.c |    2 +-
src/bin/scripts/droplang.c   |    2 +-
src/bin/scripts/reindexdb.c  |    4 +-
src/bin/scripts/vacuumdb.c   |    2 +-
src/include/port.h           |    4 ++
src/port/Makefile            |    2 +-
src/port/username.c          |   84 ++++++++++++++++++++++++++++++++++++++++++
src/tools/msvc/Mkvcbuild.pm  |    4 +-
19 files changed, 129 insertions(+), 157 deletions(-)