Implement memory caching for umove* functions

Implement memory caching for umove* functions

When the data to be fetched by vm_read_mem resides in a single memory
page, fetch the whole page and cache it.  This implementation caches
up to two memory pages.

* defs.h (invalidate_umove_cache): New prototype.
* strace.c (next_event): Call invalidate_umove_cache.
* ucopy.c (cached_idx, cached_raddr): New static variables.
(process_read_mem): New function.
(vm_read_mem): Use them.  Implement fetched page caching.
* tests/umovestr_cached.test: New test.
* tests/Makefile.am (MISC_TESTS): Add umovestr_cached.test.
* tests/umovestr_cached.c: New file.
* tests/pure_executables.list: Add umovestr_cached.
* tests/.gitignore: Likewise.