vis: handle file truncation more gracefully

If we use the file / virtual memory system as cache (using mmap(2))
and another process truncates the file we are editing, we have a
problem. All we can do is catch the resulting SIGBUS, close the
corresponding window and print a warning message.

To test this use:

 $ dd if=/dev/zero of=TEST bs=8M count=1
 $ vis TEST
   :! echo TRUNCATE > TEST

1 files changed, 3 insertions, 0 deletions

diff --git a/text.h b/text.h
index c322a10..89a5d9b 100644
--- a/text.h
+++ b/text.h
@@ -108,6 +108,9 @@ size_t text_history_get(Text*, size_t index);
 size_t text_size(Text*);
 /* query whether the text contains any unsaved modifications */
 bool text_modified(Text*);
+/* query whether `addr` is part of a memory mapped region associated with
+ * this text instance */
+bool text_sigbus(Text*, const char *addr);
 
 /* which type of new lines does the text use? */
 enum TextNewLine {