gps_ctrl.cpp: Partially rewritten, to use caching instead of throwing
away the 'wrong' messages.

The old way to do this was when a sensor was requested, to wait for the
corresponding NMEA message to pass by, throwing away all the 'wrong'
ones in the meantime. However, this leads to problems if the user
requests combinations that are based on the same NMEA string.
To avoid this we now try to cache the strings for a 'freshness' period.
examples & utils: return EXIT_FAILURE and EXIT_SUCCESS respectively
instead of 0 and 1.
Merge branch 'maint'
uhd: removed installation of unit test files
uhd: unit tests generate script w/ env paths
Merge branch 'maint'
Merge branch 'maint'
Merge branch 'maint'
uhd: work on moving binaries to lib/uhd
Merge branch 'maint'
gps_ctrl: Lower the number of retries to two when waiting for the time.
Merge branch 'maint'
Merge branch 'maint'
Merge branch 'maint'
uhd: added args member to tune_request object
usrp: poke tune args into dboards that support it
Merge branch 'maint'
docs: UHD/USRP rephrasing
Merge branch 'maint'
Merge branch 'maint'