add option for OOM score adjustment

This commit adds a new --oom-score-adj option to start-stop-daemon and
supervise-daemon, as well as an equivalent SSD_OOM_SCORE_ADJ environment
variable. If either of these are specified (with the command-line
option taking precedence), then the specified adjustment value is
written to /proc/self/oom_score_adj after forking but prior to exec'ing
the daemon (at the time when nice and ionice are applied).

Additionally, per a suggestion by Mike Frysinger, the suggested values
for the SSD_NICELEVEL, SSD_IONICELEVEL, and SSD_OOM_SCORE_ADJ variables
in the example config file are now given as zeros, which are the
kernel's default values of these process knobs for the init process at
boot. Note that uncommenting any of these zero-valued suggestions will
cause SSD/SD to set the corresponding process knob affirmatively to
zero, whereas leaving the variable unset (and the equivalent command-
line option unspecified) means SSD/SD will not change the corresponding
process knob from its inherited value.

See: https://github.com/OpenRC/openrc/pull/435#discussion_r688310672
This fixes #435.

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