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(-)