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.

bfd/cache.c

@@ -67,11 +67,11 @@ enum cache_flag {
 /* The maximum number of files which the cache will keep open at
    one time.  When needed call bfd_cache_max_open to initialize.  */
 
-static int max_open_files = 0;
+static unsigned max_open_files = 0;
 
 /* Set max_open_files, if not already set, to 12.5% of the allowed open
    file descriptors, but at least 10, and return the value.  */
-static int
+static unsigned
 bfd_cache_max_open (void)
 {
   if (max_open_files == 0)
@@ -115,7 +115,7 @@ bfd_cache_max_open (void)
 
 /* The number of BFD files we have open.  */
 
-static int open_files;
+static unsigned open_files;
 
 /* Zero, or a pointer to the topmost BFD on the chain.  This is
    used by the <<bfd_cache_lookup>> macro in @file{libbfd.h} to
@@ -176,6 +176,7 @@ bfd_cache_delete (bfd *abfd)
   snip (abfd);
 
   abfd->iostream = NULL;
+  BFD_ASSERT (open_files > 0);
   --open_files;
   abfd->flags |= BFD_CLOSED_BY_CACHE;