Add a ``log'' command for logging specific information.

Add a ``log'' command for logging specific information.
Add an ``UPTIME'' variable to indicate the bundle uptime.

It's now possible to put something like this in ppp.linkdown
for a server setup:

  MYADDR:
    log Session closing: User USER, address HISADDR, up UPTIME

Fixed some memory leakage with commands that expand words.
Made some functions static.
Fixed a diagnostic bug (iface add .... SIOCDIFADDR)