perf evsel: Rename config_callgraph() to config_callchain() and make it public

perf evsel: Rename config_callgraph() to config_callchain() and make it public

The rename is for consistency with the parameter name.

Make it public for fine grained control of which evsels should have
callchains enabled, like, for instance, will be done in the next
changesets in 'perf trace', to enable callchains just on the
"raw_syscalls:sys_exit" tracepoint.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Milian Wolff <milian.wolff@kdab.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-og8vup111rn357g4yagus3ao@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>