davfs2: fix musl compatibility

davfs2: fix musl compatibility

 - Add header tests for `error.h`, `sys/types.h` and `sys/select.h`
 - Add function tests for `canonicalize_file_name()` and `fopencookie()`
 - Add `compat.h` header to provide replacements for `error()`,
   `error_at_line()` and `canonicalize_file_name()` as well as the
   `_PATH_MOUNTED` and `_PATH_MNTTAB` defines
 - Add missing includes for `sys/select.h` and `sys/types.h`
 - Disable libneon debugging if no `fopencookie()` implementation is
   available

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>