usdt: fix attaching to USDT on Bionic Beaver

fix hist() bars

add kaddr() cal

bpftrace was using an approach different from bcc to calculate the
offset of an USDT probe. We were using only `bcc_usdt_get_location`,
whereas bcc is using a combination of `bcc_usdt_get_location` and
`bcc_resolve_symname`. Since we already use `bcc_resolve_symname` in
`AttachedProbe::offset()`, we just need to call this method to get the
same behavior from bcc.

Fixes: https://github.com/iovisor/bpftrace/issues/60

update docs to work past kprobe:sys_ changes

M capitalized

print newline for empty maps

add ctx builtin

Docs: add probes and internals diagrams

lhist human readable units

lhist truncate zero ranges

docs: add reference guide and one-liner tutorial

add rand builtin