Replace uses of timeofday() with tvgettime()

[krader1961]

While reviewing PR #1327 I noticed that we now assume gettimeofday() is available. The code that currently uses the timeofday() macro should be using tvgettime() from libast; i.e., src/lib/libast/tm/tvgettime.c.

[krader1961]

There is only one use of tvgettime(): in tvtouch(). And tvtouch() is only used one place: hist_touch(). We're probably better off eliminating that (indirect) use of tvgettime() rather than adding additional uses.