Commit 65afd075 fixed mincore() to recognize
unmapped addresses. However, it used mmu::ismapped() which just checks for
mmap()'ed addresses, and doesn't know about malloc()ed memory. This causes
trouble for libunwind (which we use for backtrace()) which tests mincore()
on an on-stack variable, and for non-pthread threads, this stack might be
malloc'ed, not mmap'ed.

So this patch adds mmu::isreadable(), which checks that a given memory range
is all readable (this memory can be mmapped, malloced, stack, whatever).
mincore() now uses that.

mmu::isreadable() is implemented, following Avi's idea, by trying to read,
with safe_load(), one byte from every page in the range. This approach is
faster than page-table-walking especially for one-byte checks (which all
libunwind uses anyway), and also very simple.