shared: move _nm_utils_ascii_str_to_int64() to "shared/nm-shared-utils.h"

shared: move _nm_utils_ascii_str_to_int64() to "shared/nm-shared-utils.h"

_nm_utils_ascii_str_to_int64() was declared in libnm-core's internal
header "nm-core-internal.h" and thus available for libnm-core, libnm,
NetworkManager and related.

It also means, the function was not available in libnm-util, libnm-glib,
clients or dispatcher. So, we either reimplemented it (nmc_string_to_int_base)
or struggle with the awkward strtol* API.