loader: open file list should be dynamic

Summary:
Open file list is currently created as statically allocated array (64 items).
Once this array is filled up, loader will not be able to operate with files.
In most cases, this mechanism is good enough, but the problem appears, when
we have many disks with zfs pool(s). In current loader implementation, all
discovered zfs pool configurations are kept in memory and disk devices open -
consuming the open file array. Rewrite the open file mechanism to use
dynamically allocated list.

Reviewed by: imp
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D31364