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
This is to avoid redefining prototypes from the libc headers. Also,
simplify the autoconf function checks and remove some copy/paste
errors checking for 'write'.
ok wouter@