Bug 11630: improve parsing of age restriction markers

Bug 11630: improve parsing of age restriction markers

This patch makes the parsing of AgeRestrictionMarker values consider
the case where the marker is immediately followed by the age, e.g.
"K16" in Finland.

How I tested:

[1] Configure Age Restricition (see Syspref AgeRestrictionMarker) and
have a biblio record with e.g. PEGI 99 in age restriction field.
[2] Try to check out to a patron with age < 99
[3] Check out should be blocked
[4] Change entry in age restriction field to PEGI99
[5] Checkout should be possible
[6] Apply patch
[7] Checkout schould now be blocked

Signed-off-by: Marc Véron <veron@veron.ch>
Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>