mmap_cache: add function to enable mmap_cache

mmap_cache: add function to enable mmap_cache

mmap_cache was enabled indirectly via unwind feature.  As now mmap_cache
can be used by other part of strace, a way to enable mmap_cache directly
is needed.

* defs.h (mmap_cache_enable, mmap_cache_is_enabled): New function
prototypes.
* mmap_cache.c (use_mmap_cache): New file local variable.
(mmap_cache_enable, mmap_cache_is_enabled): New functions.
* syscall.c (syscall_exiting_decode): Use mmap_cache_is_enabled()
instead of stack_trace_enabled.
* unwind.c (unwind_init): Invoke mmap_cache_enable.

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