git: ignore errmsg.sys and typescript, better diff header for C/C++ files
cleanup: remove hidden I_S.VARIABLES and I_S.STATUS tables

their only purpose was to distinguish between
SHOW and SELECT and there are cleaner ways of doing  it.
small cleanup
cleanup: sysvar, SHOW_VALUE_IN_HELP->GETOPT_ONLY_HELP

replace flag SHOW_VALUE_IN_HELP (that's not really an
independendent flag, as it should only be used for
NO_CMD_LINE sysvars) with a special getopt.id type.
cleanup: sysvar, only one common check_update_type()
sys_var_pluginvar: populate my_option and misc cleanup
cleanup: more 'const' qualifiers
cleanup: sql_show.cc
correct fix for the old Bug#39955 (warnings in I_S.VARIABLES)

old fix only generated a warning for the *first* row
in the output
cleanup: move safe_str*() from sql_acl.cc to m_string.h
cleanup: option_type -> scope
cleanup: VARIABLE_VALUE column should be NOT NULL

it can never be set to NULL, so it should
not use MY_I_S_MAYBE_NULL flag either
cleanup: simplify sys_var::val* methods, introduce val_str_nolock()
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
cleanup: move some reusable functionality out of show_status_array()
cleanup: split show_status_array()

move show sysvar functionality from show_status_array()
into a separate show_sysvar_array() function.
remove SHOW_SYS.

Now:

show_status_array() does not need to know about
* SHOW_SYS
* charsets
* LOCK_global_system_variables mutex

show_sysvar_array()
* gets the list of sysvars directly,
  not wrapped in SHOW_VAR
* does not need to process SHOW_xxx_STATUS values,
  neither SHOW_FUNC or SHOW_ARRAY
* not recursive
* does not have a switch over show_type at all
* reuses var->val_str(), instead of duplicating it

as a result all sysvars are always printed identically,
in all slatements.
MDEV-6138 show sysvar's help in I_S tables

SESSION_VARIABLE_PROPERTIES and GLOBAL_VARIABLE_PROPERTIES