The under 32 constraint is silly; it's to check for duplicate curves in
library-supplied configuration. That API is new as of 1.0.2. It doesn't seem
worth bothering; if the caller supplies a repeated value, may as well emit a
repeated one and so be it. (Probably no one will ever call that function
outside of maybe test code anyway.)

While I'm here, remove the 0 constraint too. It's not likely to change, but
removing the return value overload seems easier than keeping comments about it
comments about it.

Change-Id: I01d36dba1855873875bb5a0ec84b040199e0e9bc
Reviewed-on: https://boringssl-review.googlesource.com/2844
Reviewed-by: Adam Langley <agl@google.com>