Add support for client socket side that will be used to send commands to
the daemon server socket.

This patch adds only the core support, all commands using this
functionality are coming in the following patches.

Committer notes:

Hat to patch patch it to deal with this in some systems:

  cc1: warnings being treated as errors
  builtin-daemon.c: In function 'send_cmd':  MKDIR    /tmp/build/perf/bench/

  builtin-daemon.c:1368: error: ignoring return value of 'fwrite', declared with attribute warn_unused_result
    MKDIR    /tmp/build/perf/tests/
  make[3]: *** [/tmp/build/perf/builtin-daemon.o] Error 1

And also to not leak the 'line' buffer allocated by getline(), since you
initialized line to NULL and len to zero, man page says:

  If *lineptr is set to NULL and *n is set 0 before the call,
  then getline() will allocate a buffer for storing the line.
  This buffer should be freed by the user program even if
  getline() failed.
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Alexei Budankov <abudankov@huawei.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Michael Petlan <mpetlan@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: https://lore.kernel.org/r/20210208200908.1019149-6-jolsa@kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>