Fix the "winheight" command

Fix the "winheight" command

The "winheight" command is broken.  I probably broke it in one of my
TUI refactoring patches, though I didn't track down exactly which one.

The bug is that the code does:

	  *buf_ptr = '\0';

... but then never advances buf_ptr past this point, so no window name
is seen.

This patch refactors the code a bit so that a copy of the argument
string is not needed, also fixing the bug.

A new test case is included.

gdb/ChangeLog
2019-11-19  Tom Tromey  <tom@tromey.com>

	* tui/tui-win.c (tui_partial_win_by_name): Move from tui-data.c.
	Now static.  Change type of "name".
	(tui_set_win_height_command): Don't copy "arg".
	* tui/tui-data.h (tui_partial_win_by_name): Don't declare.
	* tui/tui-data.c (tui_partial_win_by_name): Move to tui-win.c.

gdb/testsuite/ChangeLog
2019-11-19  Tom Tromey  <tom@tromey.com>

	* gdb.tui/winheight.exp: New file.

Change-Id: I0871e93777a70036dbec9c9543f862f42e3a81e5