Don’t assume mode_t fits in unsigned long

Don’t assume mode_t fits in unsigned long

* src/system.c (oct_to_env): Don’t assume mode_t fits in unsigned
long.  Do not output excess leading 1 bits.  When the mode is
zero, generate "0" rather than "00".  Use sprintf instead of
snprintf, since the output won’t be truncated; in general we don’t
use snprintf unless we want output to be truncated and truncation
is typically not GNU style.