cifs: redefine NUM_SUBAUTH constant from 5 to 15

According to several places on the Internet and the samba winbind code,
this is hard limited to 15 in windows, not 5. This does balloon out
the allocation of each by 40 bytes, but I don't see any alternative.

Also, rename it to SID_MAX_SUB_AUTHORITIES to match the alleged name
of this constant in the windows header files

Finally, rename SIDLEN to SID_STRING_MAX, fix the value to reflect
the change to SID_MAX_SUB_AUTHORITIES and document how it was
determined.

Reviewed-by: Shirish Pargaonkar <shirishpargaonkar@gmail.com>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <smfrench@gmail.com>