count: use parse_ts for parsing overhead value

count: use parse_ts for parsing overhead value

* count.c (set_overhead): Change argument type to const char *, call
parse_ts to parse it and set to overhead.
* defs.h (set_overhead): Update declaration.
* strace.c: (init) <case 'O'>: do not parse argument, pass optarg to
set_overhead call.
* tests/count.test (GENERIC, WALLCLOCK, WALLCLOCK1, HALFCLOCK): New
variables with expected patterns.
Add checks for the new -O syntax.