shellcheck identifies 3 errors in disnix-ssh-client.in

[unappetizing-morsel]

Shellcheck does static analysis to produce style, info, warning, and error findings on shell scripts, and provides generally good explanations of why a particular finding could be or is an error.

Shellcheck's online version identified 3 errors, several warnings, and many informational findings in scripts/disnix-ssh-client.in. It would be good to fix the errors, and review the warnings/info findings since shellcheck's static analysis isn't necessarily always going to be good enough to figure out when a warning/info finding could actually result in an error at runtime.

At a glance, all 3 errors look like they would be relevant to supporting whitespace in filenames & would otherwise go unnoticed.