iffe: include OS standards macros in iffe tests by default

In iffe tests, some C functions are found in system libraries, but
then are not declared by the system headers on some systems because
the expected standards macros aren't defined, causing the system
headers to hide the function declarations. This may cause warnings
about invalid implicit function declarations in some tests (which
only show up if you export IFFEFLAGS=-d1), but may also cause false
negative test results. The iffe tests should be given the same
environment that their test results are going to be used in.

libast's first-run and most central feature test, 'standards',
figures out what standards macros need to be used on the current
system to get the system headers to declare the expected
functionality. All code that links to libast depends on the header
generated by this feature test. So iffe should use this result for
the tests as well, as soon as it's available (which is early in
libast's compilation cycle).

Concrete example: on Cygwin, in src/cmd/builtin/features/pty, the
'lib ptsname' test detects ptsname(3) in the system library, but
the output{...}end block that uses the _lib_ptsname feature test
result throws an 'implicit function definition' warning because
Cygwin's stdlib.h does not define this function without the
appropriate standards macros being defined first.

src/cmd/INIT/iffe.sh:
- If ${INSTALLROOT}/src/lib/libast/FEATURE/standards is available,
  incorporate it directly into iffe's own block of compiler
  massaging macros. Do not use #include as the necessary -I flags
  are not added for every test.
- Centrally define the iffe version once, not twice.
- Update and tweak getopts self-documentation (iffe --man).
- While we're here, add macOS/Darwin's DYLD_LIBRARY_PATH to the
  supported dynamic library search variables. On macOS, this is
  normally disabled by System Integrity Protection, plus this
  distribution uses static libraries at build time, so this is for
  completeness' sake and not much else. This was ported from
  @lkujaw's fork: 48ff054429
  (thanks to @JohnoKing for pointing it out).

src/lib/libast/features/standards:
- Add a comment. This is to update the file's timestamp, ensuring
  that everything will be recompiled after this commit.

src/lib/libast/features/standards

@@ -1,3 +1,4 @@
+# Disable default inclusion of <stdio.h> by setting stdio option to empty
 set stdio
 
 # Flag systems that omit necessary definitions such as u_long