kill: optimizations for single-applet build

kill: optimizations for single-applet build

* Fix a bug with a configuration in which the shell's kill builtin
  would be mistreated as a killall command (i.e. '-q' works, and
  'kill process_name' succeeds when it shouldn't):

    CONFIG_ASH_JOB_CONTROL=y
    CONFIG_HUSH_KILL=y
    # CONFIG_KILL is not set
    CONFIG_KILLALL=y
    # CONFIG_KILLALL5 is not set

* Optimize out unneeded code when the relevant applets are not
  selected.

* Move kbuild lines about shells' kill builtins from Kbuild.src to
  kill.c, to accompany the new HAVE_SH_KILL macro. I hope this would
  make maintanence a little bit easier.

Signed-off-by: Kang-Che Sung <explorer09@gmail.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>