Compiler masochism compliance.

Compiler masochism compliance.

Be able to build most of Exim with:
  -Werror -Wwrite-strings -Wunused-function -Waddress -Wpointer-sign
  -Wformat -Wuninitialized -Winit-self

Skipped a change to auth-spa which I was uncertain of. That is not
the most readable of code.

Temporarily gave up on src/src/pdkim/pdkim.c, as header_name_match()
treats the second param as const or not depending on the third param.
(I hacked the build-*/pdkim/Makefile to continue past this)

Much of this change is const propagation.