Remove a paragraph from README/Alternatives/macOS section

[bdd]

In README's "Alternatives" section, regarding to the use of ssh-agent, the paragraph about macOS assumes the need for a third-party PKCS#11 library and reasonably talks about its UX issues regarding library load path allow list.

The ssh-agent that ships with macOS (which is pretty cool, as it starts on demand and is preconfigured in the environment) also has restrictions on where the .so modules can be loaded from. It can see through symlinks, so a Homebrew-installed /usr/local/lib/libykcs11.dylib won't work, while a hard copy at /usr/local/lib/libykcs11.copy.dylib will.

On the other hand, macOS already ships with a PKCS#11 support library at /usr/lib/ssh-keychain.dylib that can be used by OpenSSH. As of macOS Catalina it only supports RSA keys but otherwise is functional.

% nm -j /usr/lib/ssh-keychain.dylib | egrep '^_C_\w+'
_C_CloseSession
_C_Finalize
_C_FindObjects
_C_FindObjectsFinal
_C_FindObjectsInit
_C_GetAttributeValue
_C_GetFunctionList
_C_GetInfo
_C_GetSlotList
_C_GetTokenInfo
_C_Initialize
_C_Login
_C_OpenSession
_C_Sign
_C_SignInit

Do you think this paragraph is still pertinent? I can put a PR mentioning /usr/lib/ssh-keychain.dylib and just remove this paragraph to keep things simple.

[FiloSottile]

Oh interesting! I had never heard of it. I'll add a line, people might be interested to find out about it.