Remove top-level side effects from importing libdelete

If the afs python modue isn't installed, then when libdelete is
imported, this was getting displayed instead of the intended warning
about afs not being available:

>`No handlers could be found for logger "libdelete"`

This is because libdelete was attempting to import afs to check for
its availability at module scope, before the main script importing
libdelete had configured logging.

Fix this by changing `have_AFS` from a module-scope variable to a
memoized function so that the check occurs after logging is
configured.

Author: Jonathan Kamens

libdelete.py

@@ -15,13 +15,7 @@
 KILO = 1024
 
 logger = logging.getLogger('libdelete')
-have_AFS = True
-
-try:
-    import afs.fs
-except ImportError:
-    logger.warn("AFS support unavailable")
-    have_AFS = False
+_have_AFS = None
 
 class DeleteError(Exception):
     pass
@@ -52,10 +46,22 @@ def format_columns(items, singlecol=False, width=80):
         rv.append("".join(item.ljust(col_width + padding) for item in c))
     return "\n".join(rv)
 
+def have_AFS():
+    global _have_AFS, afs
+
+    if _have_AFS is None:
+        try:
+            import afs.fs
+            _have_AFS = True
+        except ImportError:
+            logger.warn("AFS support unavailable")
+            _have_AFS = False
+    return _have_AFS
+
 def is_mountpoint(path):
     if os.path.ismount(path):
         return True
-    if have_AFS and afs.fs.inafs(os.path.abspath(path)):
+    if have_AFS() and afs.fs.inafs(os.path.abspath(path)):
         afs.fs.whichcell(path)
         try:
             return afs.fs.lsmount(path) is not None