camellia: fix camellia_self_test failure on compilers where char is unsigned by default

Fix camellia_self_test failure when compiled on compilers
where char is unsigned by default (on gcc this mode is
enabled with -funsigned-char).

This also fixes these warnings:

  camellia.c:433: warning: comparison is always true due
  to limited range of data type
  camellia.c:437: warning: comparison is always true due
  to limited range of data type
  camellia.c:441: warning: comparison is always true due
  to limited range of data type
  camellia.c:445: warning: comparison is always true due
  to limited range of data type
  camellia.c:450: warning: comparison is always true due
  to limited range of data type
bignum: make mpi_init() and mpi_free() accept a single argument

Make mpi_init() and mpi_free() accept a single argument. This
prevents unexpected memory corruption in some cases, makes the
API simpler to use and less error prone. Before this change,
mpi_init() and mpi_free() accepted a variable-length list
of arguments that had to be terminated by NULL.