Update userland arc4random() with OpenBSD's Chacha20
crypto/heimdal/lib/roken/rand.c: crypto/openssh/config.h: Eliminate in-tree usage of arc4random_stir(). include/stdlib.h: Remove arc4random_stir() and arc4random_addrandom() prototypes provide temporary shims for transistion period. The plan is to get rid of these before 12.0-RELEASE, after a exp-run is performed. lib/libc/gen/arc4random.c: Adopt OpenBSD arc4random.c,v 1.54 with bare minimum changes. lib/libc/gen/arc4random.h: Adopt OpenBSD arc4random.h,v 1.4 but provide _ARC4_LOCK of our own. lib/libc/gen/arc4random.3: Adopt OpenBSD arc4random.3,v 1.35 but keep FreeBSD r118247 and r114444. lib/libc/gen/arc4random-compat.c: Compatibility shims for arc4random_stir and arc4random_addrandom functions to preserve ABI. Log once when called but do nothing otherwise lib/libc/gen/getentropy.c: Fold __arc4_sysctl into getentropy.c (renamed to arnd_sysctl) and remove from ib/libc/include/libc_private.h. sys/crypto/chacha20/chacha20.c: sys/crypto/chacha20/chacha20.h: Make it possible to include the kernel implementation in libc. Checked with cpp(1).
Note that the intention is to get our arc4random() updated to OpenBSD's
version first; we will revisit and explore other possible optimizations
at a later time.