Inline PL_no_symref into pp_entersub. Deprecate the visible global variable.

Inline PL_no_symref into pp_entersub. Deprecate the visible global variable.

As the core no longer needs this fixed string in more than one place, it seems
daft to go to the overhead (and cost) of making it public in case any module
wants to use it. Modules that do want to use it should provide their own inline
copy in future.

Also restore the visible global PL_no_symref back to the original format
specification (of 5.10.0 and earlier), as an extra %s has the potential to
cause SEGVs or worse if not spotted at compile time.