The C/C++ standards don't define what time_t is, only that it is
arithmetic (and real for C11, and integral for C++). It should not be
used in portable software and is only used as the return value for some
libc calls.
A common definition for time_t is int64_t, so we'll switch to that
permanently in our own APIs. System APIs will of course stick with
time_t.
uhd::get_system_time() is an abstracted way of reading back a time, and
is not UHD-specific. As such, there's no reason to keep it in the public
part of the API where we're contractually obligated not to touch it.
Instead, moving it to the internal API space.
