patch by Satoshi Nagayasu: allows appending / saving multiple

patch by Satoshi Nagayasu: allows appending / saving multiple
selected files (closes "[ 612279 ] save multiple mesg(s) in
'Save as ...'")

* AUTHORS
	add Satoshi Nagayasu
* src/summaryview.c
	summary_save_as(): allow saving selected files to
	one file
	summary_set_menu_sensitive: enable "Save As..." when
	multiple files are selected
* src/utils.[ch]
	append_file(): new function