unwind: lift up unw_flush_cache from mmap cache management code

unwind: lift up unw_flush_cache from mmap cache management code

For making mmap cache code reusable from other areas in strace
than unwind, mmap cache related code and unwind related code
should be separated.

This change is one of the steps for the separation.  It moves
unw_flush_cache function call from the core of mmap cache code
to upper level code.

* unwind.c (mmap_cache_rebuild_result): New enum.
(rebuild_cache_if_invalid): Return MMAP_CACHE_REBUILD_* instead of
a bool value.
(build_mmap_cache): Don't call unw_flush_cache here.
(unwind_print_stacktrace, unwind_capture_stacktrace): Call
unw_flush_cache here instead.

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