textual: gettextize an overlooked string

* login-utils/islocal.c: Gettexttize the usage message.

Signed-off-by: Benno Schulenberg <bensberg@justemail.net>
textual: improve an error message and a help text

* disk-utils/isosize.c: Improve grammar of error message, and in the
help text make use of the standard angular brackets for arguments.

Signed-off-by: Benno Schulenberg <bensberg@justemail.net>