runtime: fix propagation of DONE state to message blocks
gr-uhd: added tagged stream support
uhd: fixed tags_demo
uhd: changed length_tag member variable from string to pmt symbol
uhd: added notification for tagged stream preemptions/gaps
uhd: added length tag name parameter in GRC
uhd: modified tag preemption/gap handling
uhd: Added retune-by-tag and a command interface via message passing
uhd: Allow post-burst tune; tag handling less restrictive
docs: Updated uhd to reflect tag changes.
uhd: Added freq hopping example, improved tag handling in usrp_sink
uhd: Used uhd-internal fragmentation handling, bursty tx w/o time tags now no longer uses time specs
Merge branch 'master' into next
runtime: typo fix
runtime: update message port qa to account for new system port
Merge remote-tracking branch 'osh/msg_shutdown_fix'
Merge branch 'master' into next
filter: updates kernel::fft_filter_xxx documentation to better explain how it works and how to use it in other blocks.

Addresses concern raised in issue #662.
cmake: better setup for finding QWT; otherwise, it can be forced to incorrectly "find" non-existent QWT libs.
Merge branch 'maint'