Wrong bin path with chroot inside overlayfs

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From overlayfs.txt:

Symlinks in /proc/PID/ and /proc/PID/fd which point to a non-directory
object in overlayfs will not contain valid absolute paths, only
relative paths leading up to the filesystem's root. This will be
fixed in the future.

For now, as @sj-i suggested, fallback on reading /proc/<pid>/exe. This will not work for mod_php or other embedded SAPIs.

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@akademic feel free to reopen if f14b2d6 does not work for your case. As @sj-i noted it will not work with mod_php, but should work for cli and php-fpm. I added a note to the readme about that bug.