**/Mamfile: Use IFFEFLAGS variable to add iffe flags like -d1

It was unreasonably hard to debug problems with iffe tests that
fail to compile where they should (particularly output{ ... }end
blocks that write esserntial headers).

In e72543a9 the problem was already somewhat mitigated by making
some of the failing output{ ... }end blocks emit #error directives
so that invalid/incomplete headers would cause an error at a
sensible point, and not a much harder to track error later.

This commit further mitigates the problem by making the Mamfiles
respect an IFFEFLAGS environmenet variable that is prefixed to
every iffe command's arguments. The typical use would be to export
IFFEFLAGS=-d1 to enable debug level 1: show compiler output for all
iffe tests. This now makes it reasonably feasible to detect
problems in the feature tests themselves.

src/**/Mamfile:
- Import IFFEFLAGS environment variable using setv.
- Prefix ${IFFEFLAGS} to every iffe command.

src/**/features/*:
- Amend the new fail error messages to recommend exporting
  IFFEFLAGS=-d1 to show the cause of the failure.

README.md, TODO:
- Updates.

TODO

@@ -1,18 +1,17 @@
 TODO for AT&T ksh93, 93u+m bugfix branch
 
+See the issue tracker:
+https://github.com/ksh93/ksh/issues
+
 ______
 Fix regression test failures:
 
-- On OpenBSD, there are 15 locale-related test failures in variables.sh.
 - There are many regression test failures on NetBSD.
 
 ______
 Fix build system:
 
 - ksh does not currently build on AIX or QNX.
-- Reimport the removed nmake. It is necessary for changes in Makefiles
-  to take effect. The machine-generated Mamfiles are now used as a fallback,
-  but they are not meant to be edited by hand.
 
 ______
 Fix currently known bugs affecting shell scripting. These are identified by