This patch covers a bunch of odds and ends referenced or revealed by
Jun T.'s commentary over the last few days.

1) Document the behavior of "typeset -n existing_var",  (via Jun T. comment)
2) Prohibit "typeset -nm pattern" because, well, it's insane.  Add test.
3) Improve doc for ${(!)ref} including ${{t!)ref} (Jun T.)
4) Fix doc for how-to unset of a named ref (Jun T.)
5) Allow "typeset +r -n ref" and "typeset +r +n ref" (Jun T.)
6) Fix "typeset -r -n ref=param" to create readonly references
7) Avoid accidental removal of PM_UNSET flag (Jun T.)  and update test
8) Fix "typeset -gn ref=value" and add a test for it
9) Add tests for read-only reference behavior
10) Fix infinite recursion when resolving scope of an unset local
named reference, add test.

RE #5:  Not explicitly called out anywhere yet (suggestions?) but
notice that parameter type conversions occur before parameter
assignments when using "typeset [options] var=val".  See K01 test.

RE #6: There is an explicit exception to the above for "typeset -r
var=val".  I'm not entirely happy with the way this had to be fixed
for -n, but it's necessary to prevent following the reference.

RE #10: Previously if a local named reference was unset and an
assignment was made to the name, resolve_nameref() could find a
reference of the same name in the enclosing scope and end up in an
infinite loop.  Aside from the loop, looking up-scope is wrong just
from expected behavior of locals.  The fix results in assigning a new
referent to the (thereafter not unset) reference.