MINOR: ssl: accept integer group IDs in curve2name

MINOR: ssl: accept integer group IDs in curve2name

Allow curve2name to convert integer TLS Supported Group identifiers
in addition to curve names. Register the converter with SMP_T_ANY to
preserve the input type, and accept only integer and string samples.

Return unknown IDs, or IDs without a name in the requested convention,
as decimal strings. Preserve the existing fallback for string inputs.

Document binary group extraction and extend regression coverage for
integer IDs, missing names, unknown IDs and values outside the TLS
identifier range. The build and converter regression test pass.