Teach podcheck about mmap(2)
perldiag for new mprotect warnings
SvGROW should un-cow under PERL_OLD_COPY_ON_WRITE

Otherwise pp_uc (and presumably other pieces of code) will end up mod-
ifying shared buffers.

Brought to you by PERL_DEBUG_READONLY_COW.
Make PERL_DEBUG_READONLY_COW and TRACK_MEMPOOL compatible

They both have headers at the beginning of the allocated memory.
PERL_TRACK_MEMPOOL is unique, in that it tries to free a pointer,
based not on what safesysmalloc returned, but on what it stored in
its own header.  So it needs to know about the extra COW header
when using pointer arithmetic to determine the pointer to pass
to safesysfree.
Skip t/re/uniprops.t under PERL_DEBUG_READONLY_COW

Under Linux, using mmap for all memory allocation stops working when we
get up to 70,000 lines of code.  So just skip it.