security/easy-rsa: update to 3.1.6

Quoting its changes:

• New commands: 'inline' and 'x509-eku' (#993) inline: Build an inline file for a commonName x509-eku: Extract X509v3 extended key usage from a certificate
• Expose serial-check, display-dn, display-san and default-san to command line. (#980) (Debugging functions, which remain undocumented)
• Expand default status to include vars-file and CA status (#973)
• sign-req: Allow the CSR DN-field order to be preserved (#970)