Introduce mmap_cache.h

Introduce mmap_cache.h

Let's avoid bloating defs.h and introduce a separate header for
mmap_cache.

* defs.h (struct mmap_cache_t, enum mmap_cache_protection,
enum mmap_cache_rebuild_result, mmap_cache_enable, mmap_cache_is_enabled,
mmap_cache_invalidate, mmap_cache_delete, mmap_cache_rebuild_if_invalid,
mmap_cache_search): Move ...
* mmap_cache.h: ... to this new file.
* Makefile.am (strace_SOURCES): Add mmap_cache.h.
* mmap_cache.c: Include mmap_cache.h.
* strace.c: Likewise.
* syscall.c: Likewise.
* unwind.c: Likewise.