previously, it was pretty much random which one of these trees a given
function appeared in. they have now been organized into:

src/linux: non-POSIX linux syscalls (possibly shard with other nixen)
src/legacy: various obsolete/legacy functions, mostly wrappers
src/misc: still mostly uncategorized; some misc POSIX, some nonstd
src/crypt: crypt hash functions

further cleanup will be done later.

this commit is part of an effort to make more of busybox work
out-of-the-box.

this is needed in the long term for ABI compatibility anyway, and in
the immediate, it helps with building broken programs like GNU screen
that try to prototype the functions themselves rather than using the
header.