Reduce minimum required DBD::Firebird to v1.11.

Should work starting with DBD::Firebird v0.90 when `create_database()` was
implemented, but can't be sure without testing. So just go with the lowest
tested so far (v1.11 is included in the RedHat EPEL Yum repo).
Return full isql path when not first in path.

If we find `isql` in two places, but only the second is Firebird `isql`, then
we have to return the full path to it. Otherwise the first will be executed by
Sqitch. Ref #122.