Useless uses of utmp.h

[Chris Rees]

Hi again,

I've got Busybox to compile on FreeBSD-9 now, but to do that I needed
to knock out all occurrences of utmp.h.

My solution was to make compilation of utmp.c conditional on
ENABLE_FEATURE_UTMP, and also to make
includes of utmp.h in:

coreutils/who.c
libbb/messages.c
miscutils/last.c
miscutils/runlevel.c
miscutils/wall.c

conditional on ENABLE_FEATURE_UTMP too. They still compile on FreeBSD
without the includes (and with
ENABLE_FEATURE_UTMP not defined), but does it still work on Linux afterwards?

The patch is http://www.bayofrum.net/~chris/patches/busybox-utmp-safe.diff

Perhaps it should have a little review first.

Thanks for all your help!

Chris