psql line number reporting from stdin

There is a long-standing oddity in psql that running

psql -f foo.sql

returns error messages with file name and line number, like

psql:foo.sql:1: ERROR: syntax error at or near "foo"

but running

psql < foo.sql does not. I suggest we change the latter to print

psql:<stdin>:1: ERROR: syntax error at or near "foo"

Other examples for the use of the spelling "<stdin>" in this context
include gcc and slonik.

Error messages printed in interactive mode will not be affected, of
course.

Patch attached.