Change Item_func_get_system_var::val_xxx functions to use that.

mysql-test/t/variables.test:
  @@GLOBAL.INIT_CONNECT is never NULL anymore. Nor it ever was.
  It was a bug that empty string system variables appeared as NULL
  in the *integer context* (but not in a string, real, or decimal context!)

instead of "used storage engine" and similar changes.

implement ./mtr --dry-run

not part of the SE API, and, again, mutually exclusive with
C_ORDINARY_CREATE and C_CREATE_SELECT.

allocated by the same allocator as in TABLE_SHARE::read_frm_image()

eliminating reduncancy and a possibility of setting them to a pair of
invalid values.

* persistent table versions in the extra2
* ha_archive::frm_compare using TABLE_SHARE::tabledef_version
* distinguish between "important" and "optional" extra2 frm values
* write engine-defined attributes (aka "table options") to extra2, not to extra,
  but still read from the old location, if they're found there.

use it to test main test cases with need_full_discover_for_existence > 0

Now CREATE TABLE does not write the frm file on disk,
if the engine can discover it

(in partition_archive.test frm size goes down 16 times :)

(allocating and freeing buffers on the way), allocate
one frm buffer, prepare the frm image completely in memory,
and then write it down.

* comments
* cosmetic changes, *(ptr+5) -> ptr[5]
* a couple of trivial functions -> inline
* remove unused argument from pack_header()
* create_frm() no longer creates frm file (the function used to prepare and
  fill a memory buffer and call my_create at the end. Now it only prepares
  a memory buffer). Renamed accordingly.
* don't call pack_screen twice, go for a smaller screen area in the first attempt
* remove useless calls to check_duplicate_warning()
* don't write unireg screens to .frm files
* remove make_new_entry(), it's basically dead code, always calculating
  and writing into frm the same string value. replace the function call
  with the constant string.

from everywhere - now RENAME, SHOW FULL TABLES, and TRUNCATE work with discovery.
improve error messages in truncate

SELECT * FROM `t-1` will no longer read "t-1.frm" file, use `#mysql50#t-1` for this.

"unknown" tables. Duplicates are possible - deal with them.