HOST HAS '_' IN THE HOSTNAME

Problem:
=======
'_' and '%' are treated as a wildcards by the ACL code and
this is documented in the manual. The problem with
mysql_install_db is that it does not take this into account
when creating the initial GRANT tables:

--- cut ---
REPLACE INTO tmp_user SELECT @current_hostname,'root','','Y',
'Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y',
'Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','Y','','','','',
0,0,0,0 FROM dual WHERE LOWER( @current_hostname) != 'localhost';
--- cut ---

If @current_hostname contains any wildcard characters, then 
a wildcard entry will be defined for the 'root' user, 
which is a flaw.

Analysis:
========
As per the bug description when we have a hostname with a
wildcard character in it, it allows clients from several other
hosts with similar name pattern to connect to the server as root.
For example, if the hostname is like 'host_.com' then the same
name is logged in mysql.user table. This allows 'root' users
from other hosts like 'host1.com', 'host2.com' ... to connect
to the server as root user.

While creating the intial GRANT tables we do not have a check
for wildcard characters in hostname.

Fix:
===
As part of fix escape character "\" is added before wildcard
character to make it a plain character, so that the one and
only host with the exact name will be able to connect to the
server.