remove (mk)swap(on)

they idled in KEEP to be immediately usable on a certain aboriginal
release that lacked them.
since aboriginal re-added those commands, we can now abandon them.
add own su implementation

it turned out that busybox' su pulled in a lot of code from different
TUs, so it cannot easily be said if its attackable or not.
thus i implemented my own minimal su. it's been proof-read twice
by dalias, (and much more often by me) so it should be safe.
but feel free to study it as well and report any bug you see.

it's maintained separately in my "hardcore-utils" repo, but copied
into KEEP for simplicity.

solves half of #99
closes #32
busybox: don't build standalone su
bump attr
libcap2: remove hardcoded bash assumption
busybox: restore ping's ipv6 support