bug#21707: include-file cleanup for src directory

[Paul Eggert]

Eli Zaretskii wrote:
> Would it help if we avoid including any of our headers in any
> other of our headers, so that the headers included by a particular C
> file are visible by just looking at that single C file?

That would run afoul of a more important design goal, which is that we should be 
able to include headers in any order. And anyway, compiling a typical C file 
ordinarily brings in oodles of system headers that the C file doesn't explicitly 
ask for, so regardless of our style the only practical way to see what headers a 
C file includes is to compile it and see what gcc -E outputs.

Ideally a C source file should include all headers that define symbols the 
source file directly uses, and no headers other than that. This should be true 
for both .h and .c files. We're not there by a long shot, but that should be the 
goal.