platform.h: undef HAVE_STRCHRNUL only on correct versions of FreeBSD

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
grep: fix two bugs with -w

Unfortunately, with !EXTRA_COMPAT, "grep -w ^str" still erroneously matches "strstr".

function                                             old     new   delta
grep_file                                           1499    1510     +11

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
ash: in bash compat mode, always export $SHLVL

function                                             old     new   delta
ash_main                                            1437    1442      +5

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>