Improve alignment of "info threads" output, align "Target Id" column

Improve alignment of "info threads" output, align "Target Id" column

It's long annoyed me that "info threads"'s columns are misaligned.

Particularly the "Target Id" column's content is usually longer than
the specified column width, so the table ends up with the "Frame"
column misaligned.  For example, currently we get this:

 (gdb) info threads
   Id   Target Id         Frame
   1    Thread 0x7ffff7fb5740 (LWP 9056) "threads" 0x00007ffff7bc28ad in __pthread_join (threadid=140737345763072, thread_return=0x7fffffffd3e8) at pthread_join.c:90
   2    Thread 0x7ffff7803700 (LWP 9060) "function0" thread_function0 (arg=0x0) at threads.c:90
 * 3    Thread 0x7ffff7002700 (LWP 9061) "threads" thread_function1 (arg=0x1) at threads.c:106

The fact that the "Frame" heading is in a weird spot is particularly
annoying.

This commit turns the above into into this:

 (gdb) info threads
   Id   Target Id                                    Frame
   1    Thread 0x7ffff7fb5740 (LWP 7548) "threads"   0x00007ffff7bc28ad in __pthread_join (threadid=140737345763072, thread_return=0x7fffffffd3e8) at pthread_join.c:90
   2    Thread 0x7ffff7803700 (LWP 7555) "function0" thread_function0 (arg=0x0) at threads.c:91
 * 3    Thread 0x7ffff7002700 (LWP 7557) "threads"   thread_function1 (arg=0x1) at threads.c:104

It does that by computing the max width of the "Target Id" column and
using that as column width when creating the table.

This results in calling target_pid_to_str / target_extra_thread_info /
target_thread_name twice for each thread, but I think that it doesn't
matter in practice performance-wise, because the remote target caches
the info, and with native targets it shouldn't be noticeable.  It
could matter if we have many threads (say, thousands), but then "info
threads" is practically useless in such a scenario anyway -- better
thread filtering and aggregation would be necessary.

(Note: I have an old branch somewhere where I attempted at making
gdb's "info threads"-like tables follow a model/view design, so that a
general framework took care of issues like these, but it's incomplete
and a much bigger change.  This patch doesn't prevent going in that
direction in the future, of course.)

gdb/ChangeLog:
2018-06-29  Pedro Alves  <palves@redhat.com>

	* thread.c (thread_target_id_str): New, factored out from ...
	(print_thread_info_1): ... here.  Use it to compute the max
	"Target Id" column width.

gdb/testsuite/ChangeLog:
2018-06-29  Pedro Alves  <palves@redhat.com>

	* gdb.threads/names.exp: Adjust expected "info threads" output.