agent: Fix deadlock in trustlist due to the switch to npth.
* agent/trustlist.c (clear_trusttable): New. (agent_reload_trustlist): Use new function. (read_trustfiles): Require to be called with lock held. (agent_istrusted): Factor all code out to ... (istrusted_internal): new. Add ALREADY_LOCKED arg. Make sure the table islocked. Do not print TRUSTLISTFLAG stati if called internally. (agent_marktrusted): Replace calls to agent_reload_trustlist by explicit code.
In contrast to pth, npth does not use recursive mutexes by default.
However, the code in trustlist.c assumed recursive locks and thus we
had to rework it.