libX11: Reallocarray and other memory allocation cleanups (!4)

[Alan Coopersmith]

I started converting Xlib to use reallocarray a few years back and made good 
progress, along with cleaning up some other issues I found along the way, before 
putting it aside to work on other things. Instead of letting this rot in my 
local repo, might as well merge it upstream, as it is an improvement over what 
we have now.

Review comments welcome on the merge request:
https://gitlab.freedesktop.org/xorg/lib/libx11/merge_requests/4

-- 
	-Alan Coopersmith-               alan.coopersmith at oracle.com
	 Oracle Solaris Engineering - https://blogs.oracle.com/alanc