Fix warnings in fs.c with --enable-cache-bypass

Fix warnings in fs.c with --enable-cache-bypass

Fix some warnings in src/venus/fs.c so we can compile with
--enable-cache-bypass and --enable-checking:

 -- Include ctype.h so isdigit gets a prototype

 -- Make BypassThresholdCmd have the proper signature

 -- Remove the unused 'size' variable

Change-Id: I09afc6c04c990476226d1c2a751e23d3d39085b6
Reviewed-on: http://gerrit.openafs.org/1025
Tested-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>