Move two global flags to tracee scope

Move two global flags to tracee scope

A simultaneous use of -p option and tracing of a command available
since commit v4.11-183-gfa8c286 introduces a race condition because
the flags whether the first exec has happened are global.

Fix the race by moving hide_log_until_execve and hide_log_until_execve
global variables to TCB_HIDE_LOG and TCB_SKIP_DETACH_ON_FIRST_EXEC bits
in struct tcb.flags, correspondingly.

* defs.h (TCB_HIDE_LOG, TCB_SKIP_DETACH_ON_FIRST_EXEC, hide_log):
New macros.
(hide_log_until_execve): Remove prototype.
* strace.c (skip_one_b_execve, hide_log_until_execve): Remove.
(startup_child): Set TCB_HIDE_LOG and TCB_SKIP_DETACH_ON_FIRST_EXEC bits
in the allocated tcb structure.
(init): Remove initialization of hide_log_until_execve and
skip_one_b_execve.
(print_stopped): Use hide_log() instead of hide_log_until_execve.
(trace): Check and clear TCB_SKIP_DETACH_ON_FIRST_EXEC flag instead
of skip_one_b_execve.
* syscall.c (trace_syscall_entering): Clear TCB_HIDE_LOG flag instead of
hide_log_until_execve.
(trace_syscall_entering, trace_syscall_exiting): Check hide_log()
instead of hide_log_until_execve.