Nickname defaults to system's username not IRC username

"username" alone is ambiguous and without jumping to ENVIRONMENT
explaining the use of USER, catgirl's user- and nickname options read
like pointing at each other:

-n nick | nick = nick
        Set nickname to nick.  The default nickname is the user's name.
[...]
-u user | user = user
        Set username to user.  The default username is the same as the
        nickname.

Clarify that `-n' does *not* default to `-u's value.

catgirl.1

@@ -290,7 +290,9 @@ Log chat events to files in paths
 .It Fl n Ar nick | Cm nick No = Ar nick
 Set nickname to
 .Ar nick .
-The default nickname is the user's name.
+The default nickname is
+the value of the environment variable
+.Ev USER .
 .
 .It Fl o
 Print the server certificate chain