FS#1008 - Teach procd about mounpoint or path dependencies

[openwrt-bot]

nwf:

So far as I have been able to discern, there is no mechanism for blocking a service's start until a particular filesystem has been mounted (or directory exists). The motivation is that I run collectd (via luci-statistics, natch) on my router and persist the data sets (rrds) across reboot by having them written to a USB stick. However, early in boot, collectd sometimes (often? always?) manages to come up before the mount completes, and so touches the on-router flash, creating its basedir and datadir and the rrds and so on.