chasquid: Return error on inaccessible private key file

In commit 3f9d0c0762, we added a new possible private file. If the file
exists but there's an error reading it (common for permission issues),
currently we just move on to the next file, and if it doesn't exist we
may give up with that error.

That can "hide" the access problems making them harder to debug.

To help with those kinds of errors, this patch updates the logic to
return an error if a private key exists but is inaccessible.

It also adds an unit test for this function to cover the different
scenarios, and updates the docs to match.

docs/man/chasquid.1

@@ -58,7 +58,7 @@
 .\" ========================================================================
 .\"
 .IX Title "chasquid 1"
-.TH chasquid 1 2023-10-03 "" ""
+.TH chasquid 1 2026-09-27 "" ""
 .\" For nroff, turn off justification.  Always turn off hyphenation; it makes
 .\" way too many mistakes in technical documents.
 .if n .ad l
@@ -134,11 +134,12 @@ Certificates for this domain.
 Certificate (full chain).
 .IP \fIcerts/mx.example.com/privkey.pem\fR 8
 .IX Item "certs/mx.example.com/privkey.pem"
-Private key.
+Private key (can also be named \fIkey.pem\fR).
 .PP
 Note the \fIcerts/\fR directory layout matches the one from certbot (client for
 Let\*(Aqs Encrypt CA), so you can just symlink \fIcerts/\fR to
 \&\fI/etc/letsencrypt/live\fR.
+The layout from the NixOS ACME module (\fI/var/lib/acme\fR) is also supported.
 .PP
 Make sure the user you use to run chasquid under ("mail" in the example
 config) can access the certificates and private keys.