perf beauty open: Detach the syscall_arg agnostic bits from the flags formatter

perf beauty open: Detach the syscall_arg agnostic bits from the flags formatter

We may want to use this in other contexts, like when formatting the
return of fcntl(fd, F_GETFL).

Make it have the following signature, so that we can set the formatter
for the return argument while processing the arguments of a syscall, as
fcntl, for instance, may return fds, flags, etc, so need different
return value formatters:

	size_t formatter(unsigned long value, char *bf, size_t size);

This gets so detached from 'perf trace' internals that we may well get
all these and move to a tools/lib/syscall_beauty/ library at some point
and use it in other tools/ living utilities.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexei Starovoitov <ast@kernel.org>
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-9aw8t22ztvnkuv26l6sw1c18@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>