Don't hang if no stdin or file provided

[arbovm]

If the yopass command is executed without providing --file or piping
data via stdin, the command would just hang.
This change checks if stdin can be read from via checking ModeCharDevice.

[codecov]

Codecov Report

Merging #671 (844cf6b) into master (228b3b6) will increase coverage by 0.42%.
The diff coverage is 93.75%.

@@            Coverage Diff             @@
##           master     #671      +/-   ##
==========================================
+ Coverage   78.38%   78.81%   +0.42%     
==========================================
  Files           6        6              
  Lines         347      354       +7     
==========================================
+ Hits          272      279       +7     
  Misses         49       49              
  Partials       26       26              

Impacted Files	Coverage Δ
cmd/yopass/main.go	71.96% <93.75%> (+1.96%)	⬆️

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[arbovm]

As codecov is happy now, I'm happy.

[jhaals]

@arbovm I'm also very happy with this improvement! 🙌
Thanks a lot for the contribution!

[grobie]

FWIW, the POSIX default is to attach stdin to your TTY. Just typing yopass allowed the user to type the plaintext in their terminal. The standard termination EOF state is commonly signaled in terminals via Ctrl + d. Apparently that part of POSIX / shells is not that commonly known, but it's the standard behavior of many unix commands (.e.g. cat, tail, gpg, and so many more). The decsriptions of TestDetectsNoStdinInput and similar in this change are technically wrong: /dev/tty is the stdin input attached by default, so there is stdin input which might eventually end with EOF.

With this change, the common UNIX behavior can be restored by using cat | yopass though.