checksec: more forgiving when passed directory arguments

I often use `checksec *` to lazily avoid typing
filename in a directory. If the directory contains any other
sub-dirs, the command fails. With this patch, checksec will
silently skip dir paths. There is TOCTOU issue but I don't think
checksec do anything important enough to explicitly use try/catch
to account for that.

pwnlib/commandline/checksec.py

@@ -15,20 +15,19 @@
 parser.add_argument(
     'elf',
     nargs='*',
-    type=argparse.FileType('rb'),
     help='Files to check'
 )
 parser.add_argument(
     '--file',
     nargs='*',
     dest='elf2',
     metavar='elf',
-    type=argparse.FileType('rb'),
     help='File to check (for compatibility with checksec.sh)'
 )
 
 def main(args):
-    files  = args.elf or args.elf2 or []
+    files = args.elf or args.elf2 or []
+    files = [open(f, 'rb') for f in filter(os.path.isfile, files)]
 
     if not files:
         parser.print_usage()