supervise-daemon: Fix off by ones when checking for RC_* envvars
supervise-daemon: Fix off by ones when checking for RC_* envvars

Fixes strncmp size argument off by one as the current implementation didn't
match the `=` character.

This fixes #361.

williamh/openrc,OpenRC/openrc,williamh/openrc,OpenRC/openrc,OpenRC/openrc,williamh/openrc