*: change execl sentinels from NULL to (char*)0

Signed-off-by: Rich Felker <dalias@aerifal.cx>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
cryptpw: do not segfault on EOF. Closes 6350

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