Many small changes to procfs.c to add base support for a new "info proc"

Many small changes to procfs.c to add base support for a new "info proc"
command that prints /proc specific information, changes to solib.c to
allow more flexible handling of finding the dynamic linker structures,
changes to utils.c and defs.h to add a new null_cleanup() function to
serve as an anchor point for cleanup chains with no obvious "first
cleanup".