Fix xt/subroutine-in-inc.t
Move xt/subroutine-in-inc.t to t/
Refactor environment values building

Introduce sub _env_list_value() to put all join($Config{path_sep}) and
existing value preservation in a single place.
Use that function everywhere: the code for the value for each environment
variable is now clearer.
All tests pass.
rename %_env_list_value_defaults to %ENV_LIST_VALUE_DEFAULTS
Refactor PERL_M[BM]_OPT value when --deactivate

Simpler code for setting PERL_MB_OPT and PERL_MM_OPT when a directory is
deactivated.
Patch for Carp version mismatch
Remove prototype causing warnings

... also it's a prototype.