[BusyBox] ifup, ifdown applets

[Glenn McGrath]

Ive just commited those applets based on the debian ifupdown package.

I had to do a lot of tidying, and there is still more to do, such as
  - remove long option support,
  - possibly remove the --no-mapping option, or is it usefull ?
  - there is a function in ifupdown.c called popen2()
     - it parses a variable argument list that isnt used (but i dont
understand va_* stuff)
     - i think its duplicating some of the functionality of the glibc
popen() function call
     - it uses fork and pipe and signals which making it pretty heavy
     - i just dont like it !
  - its got its own file parsing routines that really should be merged
with some libbb functions.
  - it calls ifconfig via the shell, it would save some space doing it
internally

Currently it adds about 11kB to a standard build, the upstream version is
22kB, but i still think it should be a fair bit smaller, its really only a
fancy parser that calls other applets to do the heavy work.

Any help, particuly with popen2, is most welcome.



Glenn