Also:
Simplify the check_commands()/check_startup() functions,
Provide a normal error message on missing programs,
Use '-oPubkeyAcceptedKeyTypes=+ssh-rsa' if possible,
Use the PubkeyAcceptedKeyTypes ssh option if possible,
Use /Users for MacOS /home/ replacement,
Direct stderr of more programs to /dev/null,
Use dscacheutil if possible,
Force 5-second DNS timeout for resolution,
Fix printing of the t_hostnames_chain in case of exceptional error (double_rs_chained_print()),
Pick up GitHub error message correctly (exec request failed on channel)
Simplify check_ssh_options
