Improve reporting of I/O, access errors for Emacs

[Paul Eggert]

I ran into some issues where Emacs was ignoring reasonably-serious I/O errors. I 
went through the C source code and looked for similar issues, and fixed 
everything I found. The basic idea is that Emacs should signal an error for 
serious and unexpected I/O errors (e.g., EIO, ENAMETOOLONG) while continuing to 
behave as before if the problem is merely a missing file (ENOENT) or is a 
similarly tame error. Because the patch is a bit involved and in some places 
affects MS-Windows code, I've posted it here for review:

https://bugs.gnu.org/37389

A half-dozen or so of the issues I found had simpler fixes, which I've already 
installed into master.