Maintain separate print column for each process

* defs.h (struct tcp): Add curcol.
* strace.c: (alloc_tcb): Initialize it.
(trace): Use curcol from current process and save it before
continuing.
(tprintf): Don't modify curcol on output error.
diff --git a/defs.h b/defs.h
index e5e920b..5bcaa07 100644
--- a/defs.h
+++ b/defs.h
@@ -321,6 +321,7 @@
 	long long u_lrval;	/* long long return value */
 #endif
 	FILE *outf;		/* Output file for this process */
+	int curcol;		/* Output column for this process */
 	const char *auxstr;	/* Auxiliary info from syscall (see RVAL_STR) */
 	struct timeval stime;	/* System time usage as of last process wait */
 	struct timeval dtime;	/* Delta for system time usage */