BUG/MINOR: tcpcheck: Improve LDAP response parsing to fix LDAP check

BUG/MINOR: tcpcheck: Improve LDAP response parsing to fix LDAP check

When the LDAP response is parsed, the message length is not properly
decoded. While it works for LDAP servers encoding it on 1 byte, it does not
work for those using a multi-bytes encoding. Among others, Active Directory
servers seems to encode messages or elements length on 4 bytes.

In this patch, we only handle length of BindResponse messages encoded on 1,
2 or 4 bytes. In theory, it may be encoded on any bytes number less than 127
bytes. But it is useless to make this part too complex. It should be ok this
way.

This patch should fix the issue #1390. It should be backported to all stable
versions. While it should be easy to backport it as far as 2.2, the patch
will have to be totally rewritten for lower versions.