kits/support: Skip debug check in boot loader
kernel/util: Make exit() available in kernel mode

Since we are using libraries originally intendent for user mode in kernel
mode providing them with some userland functions is inevitable. This
particular patch is to make zlib happy and able to call exit() when
its debug assertions fails.
Remove which command from default image

 * This was a script relying on a working /etc/profile, which may not be
there
 * Instead, use GNU which, available as an haikuport recipe
 * Since the command does not seem to be needed for Haiku to run (and
also because I don't know how to do it), GNU which isn't in the default
 install.
 * Typing "which" in a terminal still works, as that uses an alias
defined directly in /etc/profile.