mmap_notify: new subsystem for tracking the changes of memory mappings

mmap_notify: new subsystem for tracking the changes of memory mappings

This subsystem is a spin-off of the mmap_cache subsystem.

The mmap_cache subsystem had two features: (1) tracking the change
of memory mapping, and (2) maintaining the cache of memory mapping.

mmap_notify is a subsystem specialized to provide feature (2).
unwind-libdw itself has the feature (1).  It needs only (2).

* mmap_notify.h: New file.
* mmap_notify.c: Likewise.
* Makefile.am (strace_SOURCES): Add them.

Signed-off-by: Masatake YAMATO <yamato@redhat.com>