add <sys/filio.h> header for FIONBIO

ok beck@
miod@ fixed the "bug" so remove reference to it in the man pod^Wpage.
Remove leading underscore from _BYTE_ORDER and _{LITTLE,BIG}_ENDIAN, to be
more friendly to systems where the underscore flavours may be defined as empty.
Found the hard way be bcook@; joint brainstrom with bcook beck and guenther
Replace memset(a, 0, ...); free(a); by explicit_bzero(a, ...); free(a); in
fear a smartass-optimizing compiler decides memset is useless immediately
before free().

ok jsing@ deraadt@ tedu@
{malloc,reallocarray} + memset(,0,) -> calloc
ok tedu@