-D info in perlrun

-D info in perlrun

This changes the description of -Do to its current meaning, tweaks the
entries for -Dl and -Dr, and makes the description prefer the letters
form to the numeric one.

Note 1: The code in magic_get() sets $^D to `debug & 32767', which
loses the value of -DD. Is this intentional?

Note 2: `Label stack' doesn't seem like an especially good name for
the thing -Dl traces. `Context' matches the name of the variable
`cxstack'. If we wanted to retain the letter, it could be called the
`loop' stack, since perlsyn says `A BLOCK by itself (labeled or not)
is semantically equivalent to a loop that executes once', s///g is
like a loop, and subs and formats, um, aren't that important anyway
:-).

Credited: Ilya Zakharevich <ilya@math.ohio-state.edu>

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