Forbid using mutually exclusive options -D and -p together

Forbid using mutually exclusive options -D and -p together

If we use -D and -p option together to trace a multi-thread program, in
addition to the main thread, other threads could not be traced even if we
present -f option. Moreover, when executing 'strace -D -p <non-exist pid>',
strace could not terminate normally.

* strace.c (main): Check it.

Signed-off-by: Wang Chao <wang.chao@cn.fujitsu.com>