Added zsys_run_as () method

Drops PID into lockfile and optionally switches to specified
group/user. Ideal in combination with zsys_daemonize ().
Merge pull request #467 from hintjens/master

Added zsys_run_as () method
Fix trying to save zconfig_t to non-existant path
Merge pull request #468 from phuedx/master

Fix trying to save zconfig_t to non-existant path
zconfig_chunk_save will now save the wole config (sub)tree instead of just one config entry
Merge pull request #470 from xaqq/zconfig_chunk_save

Fix zconfig_chunk_save's abort and save the tree instead of the first entry.