checksum: Changed module interface

lr_checksum_fd(lr_ChecksumType type, int fd)
is since now:
lr_checksum_fd(lr_ChecksumType type, int fd, GError **err)

lr_checksum_fd_cmp(lr_ChecksumType type,
                   int fd,
                   const char *expected,
                   int caching)
is since now:
lr_checksum_fd_cmp(lr_ChecksumType type,
                   int fd,
                   const char *expected,
                   int caching,
                   GError **err)

Return values was changed too. Look at the comments in the header file.