No OOM error conditions in some functions of window.c

 - alloc_tabpage()
 - win_alloc_lines()
 - win_alloc_aucmd_win()
 - new_frame()
 - win_alloc()

TODO: don't handle OOM after calls to dict_alloc()
put back the undo_file_name check and use xstrdup (squash into concat_fnames)
use xstrndup in hardcopy.c (squash into prt_build_cid_fontname)
Use xmemdupz in fileio.c (squash into buf_write commit)

 - xmemdupz() is not static anymore. There are many use cases for this
	 function.