Convert C (un)signed long long values to bignums properly. This patch also does the following: - Uses make_fixnum instead of make_integer when the argument is guaranteed to be in the fixnum range. - Introduces make_unsigned_integer so that we handle unsigned values with the high bit set correctly. - Introduces conversions between bignums and (un)signed long long values. - Uses mp_set_memory_functions with the BSD MP code, if it exists. - Eliminates some unnecessary consing in the Lisp + and * implementations. - Fixes a problem with check_valid_xbm_inline(). This function is called during intialization. It calls Ftimes. When using pdump, this is a problem, because (a) the bignum code is not initialized until *after* dumping, so we don't try to dump any bignums, and (b) multiplication of integers is done inside bignums so we handle fixnum overflow correctly. I decided that an XBM file with dimensions that don't fit into fixnums is probably not something we want to try to handle anyway, and did the arithmetic with C values instead of Lisp values. Doing that broke one test, which started getting a different error message from the one it expected, so I adjusted the test to match the new reality. - Fixes a few miscellaneous bugs in the BSD MP code. See <CAHCOHQk0u0=eD1fUMHTNWi2Yh=1WgiYyCXdMbsGzHBNhdqYz4w@mail.gmail.com> in xemacs-patches, as well as followup messages.

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