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.

may not exist (the table exists only in the engine).

fixes for need_full_discover_for_existence mode

replace enum read_frm_op with a bitmap flags.
remove always-unused 'error' argument of get_table_share

Pass db and table_name into a function instead of the table_list,
when only db and table name are needed.

* rename check_if_table_exists() -> table_exists() and remove unneeded arguments

"
  revision-id: sanja@askmonty.org-20110511110948-4kdevwzomvk56y1w
  committer: sanja@askmonty.org
  branch nick: work-maria-5.1-CREATE-merge
  timestamp: Wed 2011-05-11 14:09:48 +0300
    Bugfix: New table creation/renaming block added if old encoded table present
"
the old behavior was less inconsistent than the new one.
In the new one the error message was sometimes different (under LOCK TABLES e.g.),
and there were race conditions (if this CREATE happened when a concurrent ALTER
has renamed the old table away but haven't put the new table in place)

The old one was like "(when using old table names) for DML #mysql50# prefix
is optional, for DDL it's required".

simplify open_table_def()

* print "table doesn't exist in engine" when a table doesn't exist in the engine,
  instead of "file not found" (if no file was involved)
* print a complete filename that cannot be found ('t1.MYI', not 't1')
* it's not an error for a DROP if a table doesn't exist in the engine (or some table
  files cannot be found) - if the DROP succeeded regardless

have a specially defined enum with clearly named values

and now-unused read_string() function