MINOR: ssl: add curve2name converter

MINOR: ssl: add curve2name converter

Add a string converter to normalize curve names to IANA, ANSI X9.62
or NIST notation. Match both the requested format and input names
case-insensitively, and resolve old ecdh_* aliases to canonical names.

Keep the original string unchanged when no curve matches or the
requested naming convention has no corresponding name. The converter
uses the curve table independently of the SSL library's available NIDs.

Document the supported formats and add regression coverage for curve
aliases, mixed-case formats and unchanged-string fallbacks. The build
and all three SSL curve regression tests pass.