Allow "edittrust" to accept 0 as a maximum count, and document it.

Allow "edittrust" to accept 0 as a maximum count, and document it.

Inconsistency between "addtrust" and "edittrust" reported by ZeRoFiGhter.

src/opserv.c (cmd_edittrust): Only check that argv[2] is fully parsed as
    a number; do not require it to be non-zero.
src/opserv.help: Add entry for "edittrust" and reference it appropriately.