run_command prepended the sudo password to the caller's stdin. A sudo
that does not prompt left it for the command, and fix_snmp's
'cat > snmpd.conf' wrote it as the configuration's first line before
making the file world-readable.
run_command now refuses stdin to a privileged command while a sudo
password is set. fix_snmp stages the configuration in a mktemp file
without privilege, and root installs it next to the target and renames
it over, so nothing root runs follows a symlink.
Closes#8
Christian Manivong