Fixed problem with command line arguments on 64 bit machines when
we use ~0 to indicate 'no limit'


include/m_string.h:
  Declare stpcpy() to fix portability problem on AIX with gcc 2.95.3
mysys/getvar.c:
  Fixed problem on 64 bit machines when we use ~0 to indicate 'no limit'.
sql-bench/Comments/postgres.benchmark:
  U