bfd/cache: change type used to track cached BFDs from int to unsigned

bfd/cache: change type used to track cached BFDs from int to unsigned

Within bfd/cache.c change the type for max_open_files and open_files
variables from int to unsigned.  As a consequence of this, the return
type for bfd_cache_max_open() is also changed from int to unsigned.

Within bfd_cache_max_open I've left the local 'max' variable as an
int, this should ensure that if the sysconf call fails, and returns
-1, then the computed max value will be less than 10, which means
max_open_files will be set to 10.  If 'max' was changed to unsigned
then, should the sysconf call fail, we'd end up with max becoming a
very large positive number ... which is clearly not what we want.

And, while I was auditing how open_files is used, I added an assert
within bfd_cache_delete to ensure that we don't try to reduce
open_files below zero.

There should be no user visible change with this commit.