bugs in sys_var::val_* code

1. @@boolean_var differs from SHOW VARIABLES
2. @@str_var ignored variable charset (which is wrong
   for path variables that use filesystem charset)
3. @@signed_int_var in the string context was printed
   as unsigned
mysqltest bug: replace_regex /^foo/bar/ didn't work
cleanup: @@mutex_deadlock_detector -> @@debug_mutex_deadlock_detector
MDEV-6138 show sysvar's help in I_S tables

INFORMATION_SCHEMA.SYSTEM_VARIABLES
INFORMATION_SCHEMA.SYSTEM_VARIABLES.GLOBAL_VALUE_ORIGIN
GLOBAL_VALUE_ORIGIN=AUTO