Log Message:
mail/squirrelmail: update to 1.4.23pre14997

Original changes are better support for PHP 8.2, avoiding deprecated
warnings.

pkgsrc changes

* Prefix PKGNAME with PHP_PKG_PREFIX.

* Introduce squirrelmail.mk for common definitions for other
  squirrelmail related packages.

* EGDIR and SMDIR are under PHP_PKG_PREFIX, for example:

    share/example/squirrelmail => share/example/php81/squirrelmail
    share/squirrelmail => share/php81/squirrelmail

* Use EGDIR and SMDIR in PLIST and add PRINT_PLIST_AWK support.