Better sanitation for variables fetch from pam_get_item()

[baszoetekouw]

It's unclear from the pam_get_item() documentation on what type of sanitation it does, so we can't be sure that it outputs valid strings.

So, add a wrapper function that fetches a var from pam_get_item(), copies it to a fixed-length buffer (1024 bytes should be enough for everyone!), and checks that there a no weird characters in there. At least remove ", {, and }, because they are sensitive for the output json.

[marlam]

This is related to PR #103 ; sorry that I did not see this issue earlier.

PR #103 checks the input in a separate function input_is_safe(). This could also be done in a wrapper around tty_input() or in tty_input() itself, but it does not seem to be strictly necessary for group input since that is parsed by strtol() which already catches errors, and input_is_safe() can also be used on input from other sources, e.g. the username.

I see no remaining cases of user or system input that is not validated. There is also input from the web service, but that needs to be trustworthy anyway.

[mrvanes]

'aa}bb' rejected
'aa{bb' rejected
"aa"bb" rejected
"a(*255)" accepted (MAX_INPUT_LENGTH = 256)
"a(*256)" rejected

OK