WL#8206: Introduce separate error code range for 5.7

All error messages sent from the server to the client have separate error codes.
The error code is available as the MYSQL_ERRNO item in the Condition Area. See:
  - http://dev.mysql.com/doc/refman/5.7/en/error-messages-server.html
  - http://dev.mysql.com/doc/refman/5.7/en/diagnostics-area.html 

These codes are currently numbered consecutively, starting at 1000.
We also have a second range of error codes used by the client, starting at 2000.
The problem is that the first range of codes is currently up to 1974 which
means that these two ranges will soon overlap. Which will make it hard to
differentiate between server and client errors.

This worklog is about implementing a separate range of numbers for new server
error messages introduced in 5.7. The range will start at 3000. This avoids
the problem of overlapping server and client error codes.

Further, by introducing a separate range for 5.7, it will be possible to
introduce new error codes in 5.6 even after 5.7 is GA as this can be done
without interfering with codes already in use by 5.7. (We guarantee that
a given error messages never changes its code).  We should therefore also
use separate ranges for each new GA version.

Finally, it will make it possible to e.g. reserve code ranges for forks so
that they can use codes without interfering with codes used by MySQL.

User Documentation
==================

http://dev.mysql.com/doc/relnotes/mysql/5.7/en/news-5-7-6.html

Error messages are organized in sections. In each section, error messages
are numbered consecutively.

F-1: It should now be possible to have more than one section. Each new section
     is started by adding a "start-error-number xxxx" line in errmsg-utf8.txt.
     Messages below this line will start at number "xxxx".

NF-1: errmsg.sys format should not change. This means that older versions
      (e.g. 5.5 clients) will still be able to read the 5.7 version of the file.

No other user observable changes except for that new 5.7 error messages will have
a separate number range and thus not be consecutively numbered with existing
error messages.

- Encapsulate MY_LOCALE_ERRMSGS members and move them to derror.h/derror.cc
  Especially the errmsgs strings array should be private so that the mapping
  of number to string can be controlled.
- Change the signature of my_error_register() from
     extern int my_error_register(const char** (*get_errmsgs) (), ...
  to
     extern int my_error_register(const char* (*get_errmsg) (int), ...
  so that the function provided is responsible from mapping from number
  to string.
- Keep error messages packed in a single array, but modify the lookup in
  the array from (nr - 1000) to (nr - section_start + size_of_earlier_sections).
  Example: Sections start at 1000 and 3000. Section 1 has 886 messages.
           Error nr 3567 => (3567 - 3000 + 886) = array index 1453.
- Remove ER_ERROR_FIRST and ER_ERROR_LAST #define's from mysqld_error.h
  Replace them with
    static const int errmsg_section_start[]
    static const int errmsg_section_length[]
  arrays containing the start nr and length of each section.