safe password entry for GNU R, Git, and SSH

 Cross-platform utilities for prompting the user for credentials or a
 passphrase, for example to authenticate with a server or read a protected key.
 Includes native programs for MacOS and Windows, hence no 'tcltk' is required.
 Password entry can be invoked in two different ways: directly from R via the
 askpass() function, or indirectly as password-entry back-end for 'ssh-agent'
 or 'git-credential' via the SSH_ASKPASS and GIT_ASKPASS environment variables.
 Thereby the user can be prompted for credentials or a passphrase if needed
 when R calls out to git or ssh.