_netr_ServerPasswordSet2: use info level 26 to set plain text machine password

_netr_ServerPasswordSet2: use info level 26 to set plain text machine password

To support password change for machine or trusted domain accounts in Active
Directory environment we need to pass down actual plain text password
instead of NT hashes. This would allow a backend like ipasam to update
Kerberos keys as well as NT hashes.

By calling samr_SetUserInfo2 info level 26 we ensure PASSDB layer can
actually get the plain text password. If PASSDB backend implements
pdb_update_sam_account() callback, it then gets the plain text password
from samr_SetUserInfo2.

A plain text password is a data blob represented as up to 256 WCHARs. It
is UTF-16 coded on wire and we have its length from the buffer.
SetUserInfo2 SAMR call chain in decode_pw_buffer() does explicitly
expect 512+4 bytes in the buffer. It then calls convert_string_talloc()
to convert it to UNIX charset passing the correct value of the plaintext
password length. However, convert_string_talloc() expects the length of
input string *including* the terminating null and we pass just the
string length.

convert_string_talloc() then explicitly null-terminates the resulting
string by adding two nulls. In most cases UNIX charset is UTF-8, so we
get null-terminated UTF-8 string down to PASSDB layer.

MS-SAMR does not limit what does the password should contain.  It says
it is 'userPassword' value. Either 'userPassword' or 'unicodePwd' cannot
contain null characters according to MS-ADTS 3.1.1.3.1.5 because they
must be proper UTF-8 and UTF-16 strings accordingly.

We are talking to our own SAMR service here.

Signed-off-by: Alexander Bokovoy <ab@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>