In preparation for the forthcoming -Y/--decode-pids=comm option.

Load the content of /proc/$pid/comm to `comm', a newly introduced
member of struct tcb.

The loading is done when
A. a tcb is initialized,
B. execve or execveat is called, and
C. prctl(PR_SET_NAME,...) is called.

Limitation: this change is not enough for handling the case that
someone writes a value to /proc/$pid/comm directly.
(e.g. running echo "foo" > /proc/$pid/comm)

* src/defs.h (struct tcb): New member `comm'.
(enum pid_decoding_flags): Add PID_DECODING_COMM as a new flag.
(PROC_COMM_LEN): New macro.
(maybe_load_task_comm): New function declaration.
* src/strace.c (strip_trailing_newlines): New function.
(load_task_comm): New function loading the contents of /proc/$pid/comm
into struct tcb.comm.
(maybe_load_pid_comm): New function.
(alloctcb): Call it for initializing the `comm' member.
* src/syscall.c: Include <linux/prctl.h>.
(syscall_exiting_decode): Call maybe_load_task_comm().
* src/sysent.h...