Bug#55794: ulonglong options of mysqld show wrong values.

Port the few remaining system variables to the correct mechanism --
range-check in check-stage (and throw error or warning at that point
as needed and depending on STRICTness), update in update stage.
Fix some signedness errors when retrieving sysvar values for display.

mysql-test/r/variables.result:
  Show that we throw warnings or errors depending on strictness
  even for "special" variables now.
mysql-test/t/variables.test:
  Show that we throw warnings or errors depending on strictness
  even for "special" variables now.
sql/item_func.cc:
  show sys_var_ulonglong_ptr and SHOW_LONGLONG type variables as unsigned.
sql/set_var.cc:
  move range-checking from update stage to check stage for the remaining
  few sys-vars that broke the pattern
sql/set_var.h:
  add check functions.