Investigate supporting libssh2

[davidrg]

The current libssh backend works well and supports the latest cryptography options as well as Kerberos authentication, but due to its compiler requirements it doesn't support anything older than Windows XP SP3 or Windows on Itanium.

In the past Kermit 95 supported SSH on Windows 95 and NT 4.0. It would be nice to be able to do that again, and it would be nice to finally provide SSH on OS/2 for the first time. With the compiler situation on those platforms this will probably never happen with libssh.

The best options for SSH on Vintage platforms at this time seem to be either libssh2 (no relationship with libssh), or WolfSSH (#236). Libssh2 is the most compatible license-wise (BSD), but both need a crypto backend to function. Of the crypto backends supported by libssh2 (OpenSSL, libgcrypt, mbedTLS, wolfSSL), WolfSSL seems to be the best bet given others have previously got it building with Visual C++ 2003 and even Open Watcom for 16-bit windows.

• Refactor the whole SSH subsystem into a DLL #175
• Investigate building WolfSSL with Open Watcom #364
• Write a new SSH backend using libssh. ckonssh.c can be used as a template/starting point.

See also: #236 - Investigate WolfSSH