uhd: added multi-thread build flag for msvc
usrp2: fix firmware rx clear register offset

When an ICMP dest unreachable pkt arrives,
the fw needs to know how to shutoff the DSP.
This offset for the reset register was
broken by a previous fix for register overlap.
Merge branch 'maint'
examples: changed examples that force usage of all channels to allow user to input which channels to use
test_timed_commands: explicitly set RX stream time to be less than recv timeout
Merge branch 'maint'