A reimplementation of g_file_get_contents() to overcome two limitations:

 - nm_utils_file_get_contents() accepts a @dirfd argument to open the
   file relative using openat().
 - nm_utils_fd_get_contents() allows to read the content from a file
   filedescriptor.
 - both support a max_length argument, to fail gracefully if we
   get tricked into loading a huge file.