Minor enhancements

[javier]

Below is a list of minor todo items that we've collected as we were working on the implementation of open80211s. They are small tasks well suited for those of you interested in becoming familiar with the code. Sorted in order of decreasing priority:

1. Deal with overflow on time conversions (mostly the macros on mesh_hwmp.c) or limit possible values so that they cannot overflow.
2. Precursor support (including nice lifecycle).
3. Multiple destination count in PREPs.
4. Prevent setting MeshTTL == 0 and remove the dumb checks on MeshTTL == 0 on the tx path and PREQ sending.