Turn _idn2_punycode_encode, _idn2_punycode_decode into compat symbols

These internal symbols should not be used by applications, but were previously exported. Contrary to our expectations, outright removal is not possible due to some use in old GNUTLS versions.

The aliases are required because internal hidden references turn the target symbol into a hidden symbol. It would have been possible to preserve the internal API using another asm alias, but this commit renames the call sites instead, for simplicity.

The -export-symbols-regex argument in lib/Makefile.am is not adjusted because targets which do not support version scripts probably do not have stringent backwards compatibility requirements.

Fixes #74 (closed).