Avoid attacks on sitecustomize by using NUL delimiters to wrap filenames.

Avoid attacks on sitecustomize by using NUL delimiters to wrap filenames.

Previously the generated code used regular '' strings, which meant that a
crafted pathname containing ' characters could be used to inject code.
Until the previous commit, this was only a problem if building in or
Configuring to install to such a directory. Which, hopefully, would be
"obviously wrong" to anyone capable of building Perl from source.

However, fixing the bug that prevented sitecustomize being subject to
relocatable include now means that for a relocatable pearl, an end-user
controlled path can now reach the sitecusomize code.