Don't remove newlines when parsing config, menu and XBM

Removing newlines in rc.xml and menu.xml caused parser error with
following content:

<!--
 -
 - Some comments
 -
-->

...though it is a valid XML.

Let's not do that. I moved `grab_file()` to `buf.c` and renamed it to
`buf_from_file()`, because it now directly touches `struct buf` and
I don't like having a source file only for one function.

include/common/buf.h

@@ -109,4 +109,11 @@ void buf_reset(struct buf *s);
  */
 void buf_move(struct buf *dst, struct buf *src);
 
+/**
+ * buf_from_file - read file into memory buffer
+ * @filename: file to read
+ * Free returned buffer with buf_reset().
+ */
+struct buf buf_from_file(const char *filename);
+
 #endif /* LABWC_BUF_H */