Four new attributes and some minor fixes

Hello,

attached a diff which adds (mail|post)_mime_encoding and (mail|post) 
_8bit_header to the attributes system. In addition, I have changed  
the following things:

options_menu.c:UPDATE_INT_ATTRIBUTES:
I removed the 'if (i) ...' because the code within the second if ()  
does the same.

options_menu.c:change_config_file():
UPDATE_INT_ATTRIBUTES was missing in OPT_SHOW_UNREAD_ARTS,  
OPT_THREAD_PERC, OPT_SHOW_INFO, OPT_NEWS_QUOTE_FORMAT, OPT_QUOTE_CHARS.

In OPT_SHOW_UNREAD_ARTS, OPT_SORT_TREADS_TYPE, OPT_THREAD_ARTICLES:  
group->attribute->option = tinrc.option has to be set immediately  
because make_threads() uses it.

attrib.c:SET_INT_ATTRIBUTES:
Fixed a bug I introduced. Numeric attributes got the value of the  
last scope in scopes[] which matches the group. A check was missing  
wether the value was the default or set from the attributes file.

config.c:read_config_file(), options_menu.c:change_config_file(),  
attrib.c:assign_attributes_to_groups():
I added code to adjust mutually exclusive settings (some combinations  
of (mail|post)_mime_encoding, (mail|post)_8bit_header and, in case of  
CHARSET_CONVERSION, mm_network_charset) like  
options:menu.c:change_config_file() does it in case of  
OPT_MM_NETWORK_CHARSET (for instance).

The patch is against the latest snapshot.

Regards,
Dennis