configure: renamed '--etcdir' -> '--sysconfdir'; Makefile.in: reverted unrelated fix;

 * we now use '--sysconfdir', for greater consistency with other
   'configure' scripts (as per github comment by @ib);
  * renamed (non-user-visible) variables for consistency;

 * removed a fix (to be committed to another topic branch) which made it
    possible to use a '--prefix' directory that does not exist already
    (as per github comment by @ib);
fixed: Makefile.in: specifying a nonexisting directory to '--prefix' (in 'configure') now works;
Merge branch 'feature-support_custom_etc_dir-01' (fixes from review comments by @ib)
Merge branch 'fix-install_to_nonexisting_prefix-01'