* Fixes#4875: Refactor process listing
* Move procfs-based process listing into linux_debug.c, guarded by __linux__
* Provide a warning and eprintf a TODO on the remaining platforms.
* Break reusable parts into linux_get_proc_pid and call it as needed.
* Add/remove comments for clarity
* Address feedback and re-enable non-Linux
* http://man7.org/linux/man-pages/man5/core.5.html
* Able to dump the header and the program headers
* Checks /proc/[pid]/coredump_filter to know which maps should be dumped
* Check for bit 6 to 0 (priv/share anon mappings, priv/share file-backed, elf hdr, priv/share huge pages)
* Missing: priv/share DAX pages
Since `/proc/%d/task` is directory, it make sense to use appropriate
check function. Therefore,`r_file_exists` function is not suitable
here, because it checks for existence of regular file.
