Add SRC_UPDATE_SKIP, DOC_UPDATE_SKIP, and PORTS_UPDATE_SKIP
variables.  These are intended to allow bypassing the
'svn co /usr/{src,doc,ports}' step in the chroot when the
tree exists from external means.

The use case here is that /usr/src, /usr/doc, and /usr/ports
in the chroot exist as result of zfs dataset clones, so it
is possible (and happens quite often) that the included
distributions may not be consistent.  (This is not the case
for -RELEASE builds, but does happen for snapshot builds.)

Tested on:	stable/9@r264319
MFC After:	3 days
Sponsored by:	The FreeBSD Foundation
Tune buffer sizes for SuperSpeed USB when using LibUSB v0.1 and v1.0
APIs to increase the maximum bandwidth limit.

MFC after:	1 week