perf tools: Move extra string util functions to util/string2.h

perf tools: Move extra string util functions to util/string2.h

Moving them from util.h, where they don't belong. Since libc already
have string.h, name it slightly differently, as string2.h.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-eh3vz5sqxsrdd8lodoro4jrw@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>