smbXcli: Detect the SMB311 posix negotiate context

smbXcli: Detect the SMB311 posix negotiate context

The server will only return this if the client requested in via
smbXcli_negprot_send()'s in_ctx parameter. This adds knowledge about
SMB2_CREATE_TAG_POSIX to smbXcli_base.c with a function to query
it. The alternative would have been to detect this in the caller, but
this would have meant that we also would need a
smbXcli_conn_set_have_posix() function or something similar.

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Ralph Boehme <slow@samba.org>