#490 mh-alias-system-aliases loses if nmh was installed from source

[Mike Kupfer]

If nmh is built from source, the default location of the MailAliases file is /usr/local/nmh/etc/nmh/MailAliases, which is not included in mh-alias-system-aliases. It seems like we should be using the etcdir path from mhparam for this, though I suppose that could conflict with having mh-alias-system-aliases being user customizable (which it is currently).