Ignore BOM in config files

Fixes #2823.

Also remove unnecessary feof check.

src/basic/utf8.h

@@ -28,6 +28,7 @@
 #include "missing.h"
 
 #define UTF8_REPLACEMENT_CHARACTER "\xef\xbf\xbd"
+#define UTF8_BYTE_ORDER_MARK "\xef\xbb\xbf"
 
 bool unichar_is_valid(char32_t c);