Move username lookup functions from /port to /common

src/backend/libpq/auth.c

src/backend/main/main.c

src/bin/initdb/initdb.c

src/bin/psql/help.c

src/bin/scripts/common.h

src/common/Makefile

src/common/username.c

src/include/common/username.h

src/include/port.h

src/port/Makefile

src/tools/msvc/Mkvcbuild.pm