MFC: r256968, r256971, r256978

Improve SRV records support for the pkg(8) bootstrap:
- order srv records by priorities
- for all entries of the same priority, order randomly respect the weight
- select the port where to fetch from respect the port provided in the SRV
record

Allow to bootstrap by doing pkg add ./a/path/to/a/pkg_package.txz

Approved by:	re (glebius)
MFC r256847:
Print more useful information about the transfer that trigger the assertion.

Approved by:	re (glebius)
MFC r256848:
Use plain register read for waiting of the reset completion
notification, to avoid gt_lock recursion.

Approved by:	re (glebius)