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.

src/common/buf.c

@@ -6,6 +6,7 @@
 #include <stdbool.h>
 #include <stdio.h>
 #include <string.h>
+#include <wlr/util/log.h>
 #include "common/macros.h"
 #include "common/mem.h"
 #include "common/string-helpers.h"
@@ -203,3 +204,37 @@ buf_move(struct buf *dst, struct buf *src)
 	*dst = *src;
 	*src = BUF_INIT;
 }
+
+struct buf
+buf_from_file(const char *filename)
+{
+	struct buf buf = BUF_INIT;
+	FILE *stream = fopen(filename, "r");
+	if (!stream) {
+		return buf;
+	}
+
+	if (fseek(stream, 0, SEEK_END) == -1) {
+		wlr_log_errno(WLR_ERROR, "fseek(%s)", filename);
+		fclose(stream);
+		return buf;
+	}
+	long size = ftell(stream);
+	if (size == -1) {
+		wlr_log_errno(WLR_ERROR, "ftell(%s)", filename);
+		fclose(stream);
+		return buf;
+	}
+	rewind(stream);
+
+	buf_expand(&buf, size + 1);
+	if (fread(buf.data, 1, size, stream) == (size_t)size) {
+		buf.len = size;
+		buf.data[size] = '\0';
+	} else {
+		wlr_log_errno(WLR_ERROR, "fread(%s)", filename);
+		buf_reset(&buf);
+	}
+	fclose(stream);
+	return buf;
+}