'RELENG_4_2_0_RELEASE'.

This commit was manufactured to restore the state of the 4.2-RELEASE image.
Releases prior to 5.3-RELEASE are omitting the secure/ and crypto/ subdirs.

  Understand environment variables in commands
  Understand IP6 addresses in ``show route''
  Correct an alignment bug in ``show route''
  Add the ``resolv'' command
  Add ``dns'' logging
  Rename struct descriptor -> struct fdescriptor
  Fix some printf-style parameter-type bugs
  Use <nnn> instead of <???> when something's not understood
  Log (debug) information about incoming fragment handling
  Correct two bugs in the timer code
  Add some {Net,Open}BSD compatibility stuff
  Fix some gramar and line breaks in ppp.8

Requested by: peter

      See the file README.changes, and re-read the man page.

o Shuffle things that live at the datalink level into
  ``show link'' rather than ``show modem''.
o Make both ``show'' commands prettier and more consistent,
  and display carrier status, link type and our name in
  ``show modem''.
o Show redial and reconnect information in ``show link''
  and remove ``show redial'' and ``show reconnect''.
o Down the correct link in bundle_LinkLost().
o Remove stale -direct and -background links at the end
  of our main loop, not when we know they're going.  This
  prevents unexpected pointer-invalidations...
o If we ``set server'' with the same values twice, notice
  and don't moan about failure.
o Record dial script despite our link mode.  The mode may
  be changed later (next mod) :-)  We never run scripts
  in -direct and -dedicated modes.
o Make ``set server none'' functional again.
o Correct datalink state array so that we don't report an
  ``unknown'' state.
o Pass struct ipcp to IpcpCleanInterface, not struct fsm.
o Create TUN_PREFIX define rather than hard-coding in main.c
o prompt_TtyInit now handles a NULL prompt for -direct mode
  rather than having to create one then destroy it uncleanly.
o Mention our mode in the "PPP Started" LogPHASE message.
o Bring all auto links up when we have something to send.
o Remove some redundant Physical_*() functions.
o Show which connection is running a command when logging
  commands.
o Initialise throughput uptime correctly.

o Move Var*Version into command.c
o Remove struct pppVars (and there was much rejoicing) !
o Forward-decl some structs in .h files to avoid include
  ordering requirements and remove a few more redundant
  #includes.

  o Our diagnostic socket has its password set in the `set socket'
    line only (not in ppp.secret).
  o Passwords are per server socket (*VarAuthKey are gone)
  o Authority is per prompt (VarLocalAuth is gone).
  o Local logging is per prompt.
  o Add a `show who' command to see who's connected.  No identd
    routine - just a `where the connection came from' display.
  o SIGUSR1 is disabled for now - we have no way of choosing a
    password for the socket created :-(

Prompts are attached as a list of `struct descriptor's in
struct bundle, and serviced under the bundles descriptor
service routines.  Ultimately, everything should be done
like this.

Cosmetic:
  o alphabeticalise SRCS in Makefile.
  o Add a few comments in command.h

TODO: Start checking that we don't overflow the descriptor sets
      in select() now that we can have any number of descriptors.