Loader paged/pageable data is not always paged.

This change does modify devsw dv_print() to return the int value,
enabling walkers to interrupt the walk on non zero value from dv_print().

This will allow the pager_print actually to stop displaying data on
user input, and additionally pager is used in various *dev_print callbacks,
where it was missing.

For test, lsdev [-v] command should display data by screenfuls and should
stop when the key 'q' is pressed on pager prompt.

Reviewed by: allanjude
Approved by: allanjude (mentor)
Differential Revision: https://reviews.freebsd.org/D5461