[PATCH 4/6] lib-var-expand: Change escape func signature to return int with error_r

[PATCH 4/6] lib-var-expand: Change escape func signature to return int with error_r

Wire error propagation in var_expand_program_execute_one_real() so a
failing escape function causes the expansion to return -1.

Update all implementations (passdb-sql, userdb-sql, db-ldap, dict-ldap,
auth-request-var-expand) and replace the unsafe auth_request_escape_func_t
cast in auth-request-var-expand.c with a proper bridge wrapper.

Gbp-Pq: Name 0004-lib-var-expand-Change-escape-func-signature-to-retur.patch