[Rd] Lack of protection bug in current R release candidate

[Radford Neal]

The current R release candidate has a lack of protect bug (of very
long standing) with respect to the R_print.na_string and
R_print.na_string_noquote fields of the static R_print structure
declared in Print.h.  This shows up very occassionally as incorrect
output from the following lines in reg-tests-2.R:

  x <- c("a", NA, "b")
  factor(x)
  factor(x, exclude="")

The solution (kludgy, but the whole concept is kludgy) is to forward
R_print.na_string and R_print.na_string_noquote with the other "roots"
in RunGenCollect (after the comment /* forward all roots */).

   Radford Neal