Add 'net rpc join' to match the ADS equiv.

Add 'net rpc join' to match the ADS equiv.

This kills off the offending code in smbpasswd -j -Uab%c

In the process we have changed from unsing compelatly random passwords
to random, 15 char ascii strings.  While this does produce a decrese in
entropy, it is still vastly greater than we need, considering the application.

In the meantime this allows us to actually *type* the machine account
password duruign debugging.

This code also adds a 'check' step to the join, confirming that the
stored password does indeed do somthing of value :-)

Andrew Bartlett
(This used to be commit c0b7ee6ee547dc7ff798eaf8cb63fbe344073029)