libsframe: revisit sframe_find_fre API

libsframe: revisit sframe_find_fre API

Inspite of implementing a rather simple functionality, this function was
relatively difficult to follow, and maintain.  Some changes are done now
to address that - refactor the function and use better names to make it
more readable.

The changes to the implementation do not cause any change in the
contract of the API.

libsframe/
        * sframe.c (sframe_fre_get_end_ip_offset): to here...
        (sframe_find_fre): Refactor some bits from...