Alias command has no defined charset

[davez]

When using alias command to add aliases to alias file mutt, just writes text in the current charset. So the alias file ends up with lines using different charsets, making aliases (mostly names) a mess if they were added from terminals with different charsets.

There should be a way to define charset which will be used for alias file, regardless of the current terminal chrset.

I see two ways to implement this:
1) add a new command, say alias_file_charset, which would define this.
2) when parsing config files, remember config_charset for the file specified with alias_file and then use that charset when adding aliases.

Both could be implemented, but I'd be happy with either one.

[pdmef]

(In [f93dcd689032]) Encode lines written to $alias_file in $config_charset if set. Closes #3095