Hide startup syscalls.

Hide startup syscalls.

Tested with "./strace [-D] [-q] [-bexecve] env true",
all cases seem to work.

* defs.h: Declare new variable: bool hide_log_until_execve.
* strace.c: Define hide_log_until_execve.
Rename skip_startup_execve to skip_one_b_execve.
(startup_child): Do not set skip_one_b_execve = 1 here.
(init): If "strace PROG" case (as opposed to "strace -pPID"),
set skip_one_b_execve and hide_log_until_execve to 1.
(trace): Don't print signal messages if hide_log_until_execve == 1.
* syscall.c (trace_syscall_entering):
Skip syscall printing if hide_log_until_execve == 1.
Reset hide_log_until_execve if we enter execve syscall.
(trace_syscall_exiting): Skip syscall printing if hide_log_until_execve == 1.

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>