Simplify autoconf checks by using AC_CHECK_FUNCS/HEADERS.
Clarify some ambiguous dependencies around strnlen/strndup.
Unconditionally enable pidwraptest for all arc4random implementations.
Remove HAVE_VASPRINTF conditional, since asprintf requires vasprintf.
ok @doug
The original author clarified the license, so we are fine to ship with
the PID wraparound test. Run it by default if libressl is using the
native arc4random supplied from the OS.
ok deraadt@
