Bug 10745: remove more ambiguity for translators for XSLT subfieldSelect parameters

As in Bug 6278, this patch replace ">at<" with ">a_t<"
so not to mislead translators. Found more cases that
initial description.

To test:

1) Check that problem exists and only on xslt files,
using your preferred language

egrep -B10 "^msgid \"at\"" misc/translator/po/xx-YY-i-staff-t*

2) Apply the patch

3) Update translation file
(cd misc/translator; perl translate update xx-YY)

4) Verify the problem is gone, repeat 1

5) If you like verify new entries in PO file

egrep -B15 "^msgid \"a_t\"" misc/translator/po/xx-YY-i-staff-t*

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Tested this with field 780 and running a Dutch translation.

Signed-off-by: Jared Camins-Esakov <jcamins@cpbibliography.com>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>