PL_foo_mutex is global, _depth is per interpreter

There are a couple of variable pairs,

    PL_env_mutex    + PL_env_mutex_depth
    PL_locale_mutex + PL_locale_mutex_depth

which are used as recursive locks: the depth variable holds the depth of
recursion.

For both these pairs, the mutex var is global and is defined in
perlvars.h, while the depth var is per-interpreter and is defined in
intrpvar.h.

This sounds wrong to me. Is there anything here I'm not seeing, or should
I move PL_*_mutex_depth to perlvars.h?