Bug#4098 - mod_sftp unable to use SFTPHostKey due to being group readable in
CentOS 7.
Add integration/regression test for Bug#4098, and improve the log message
when insecure permissions on hostkey files are found.
More changes (including docs) needed for implementing fix Bug#4098.
Merge pull request #44 from proftpd/sftp-hostkey-perms-bug4098

Bug#4098 -  mod_sftp unable to use SFTPHostKey due to being group readable in CentOS 7.