Convey locale-related environment variables as backup, and nicer error.

Fixes #74.

man/mosh-server.1

@@ -76,6 +76,12 @@ UDP port number to bind
 .B \-c \fICOLORS\fP
 Number of colors to advertise to applications through TERM (e.g. 8, 256)
 
+.TP
+.B \-l \fINAME=VALUE\fP
+Locale-related environment variable to try as part of a fallback
+environment, if the startup environment does not specify a character
+set of UTF-8.
+
 .SH EXAMPLE
 
 .nf

man/mosh.1

@@ -64,6 +64,18 @@ pass between client and server. By default, \fBmosh\fP uses the ports
 between 60000 and 61000, but allows the user to request a particular
 UDP port instead.
 
+\fBmosh\fP will do its best to arrange a UTF-8 character set locale on
+the client and server. The client must have locale-related environment
+variables that specify UTF-8. \fBmosh\fP will pass these client
+variables to the \fBmosh-server\fP on its command line, but in most
+cases they will not need to be used. \fBmosh-server\fP first attempts
+to use its own locale-related environment variables, which come from
+the system default configuration (sometimes /etc/default/locale) or
+from having been passed over the SSH connection. But if these
+variables don't call for the use of UTF-8, \fBmosh-server\fP will
+apply the locale-related environment variables from the client and try
+again.
+
 .SH OPTIONS
 .TP
 .B \fIcommand\fP