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

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.