Disallow control characters in name fields

Earlier versions of the spec put no limits on name fields, but
indicate that names should be usable as identifiers on storage
backends. Control characers obviously conflict with this purpose,
so specify that they're prohibitted, while making it clear that
all other valid Unicode strings are allowed.

Fixes #267

Author: bswartz

csi.proto

@@ -189,6 +189,10 @@ message CreateVolumeRequest {
   //    an identifier by which to refer to the newly provisioned
   //    storage. If a storage system supports this, it can optionally
   //    use this name as the identifier for the new volume.
+  // Any Unicode string that conforms to the length limit is allowed
+  // except those containing the following banned characters:
+  // U+0000-U+0008, U+000B, U+000C, U+000E-U+001F, U+007F-U+009F.
+  // (These are control characters other than commonly used whitespace.)
   string name = 1;
 
   // This field is OPTIONAL. This allows the CO to specify the capacity
@@ -791,6 +795,10 @@ message CreateSnapshotRequest {
 
   // The suggested name for the snapshot. This field is REQUIRED for
   // idempotency.
+  // Any Unicode string that conforms to the length limit is allowed
+  // except those containing the following banned characters:
+  // U+0000-U+0008, U+000B, U+000C, U+000E-U+001F, U+007F-U+009F.
+  // (These are control characters other than commonly used whitespace.)
   string name = 2;
 
   // Secrets required by plugin to complete snapshot creation request.