Replace struct timeval with struct timespec in time measurements

Replace struct timeval with struct timespec in time measurements

This is required to implement more precise time measurements.

* Makefile.am (strace_LDADD): Add $(clock_LIBS).
* defs.h (struct tcb): Change the type of stime, dtime, and etime fields
from struct timeval to struct timespec, all users updated.
(syscall_exiting_decode, syscall_exiting_trace, count_syscall): Change
the type of "struct timeval *" argument to "struct timespec *", all
users updated.
(tv_nz, tv_cmp, tv_float, tv_add, tv_sub, tv_div, tv_mul): Rename to
ts_nz, ts_cmp, ts_float, ts_add, ts_sub, ts_div, and ts_mul.  Change
the type of all "struct timeval *" arguments to "struct timespec *",
all users updated.
* util.c (tv_nz, tv_cmp, tv_float, tv_add, tv_sub, tv_div, tv_mul):
Rename to ts_nz, ts_cmp, ts_float, ts_add, ts_sub, ts_div, and ts_mul.
Change the type of all "struct timeval *" arguments to "struct timespec *".
* count.c (struct call_counts): Change the type of "time" field
from struct timeval to struct timespec, all users updated.
(overhead): Change type from struct timeval to struct timespec, all
users updated.
(count_syscall): Change the type of "struct timeval *" argument to
"struct timespec *".
* strace.c (printleader): Change the type of struct timeval variables
to struct timespec, call clock_gettime instead of gettimeofday.
(next_event, trace_syscall): Change the type of struct timeval variables
to struct timespec.
* syscall.c (syscall_entering_finish, syscall_exiting_decode): Call
clock_gettime instead of gettimeofday.