libperf: Adopt the readn()/writen() functions from tools/perf

Move the readn()/writen() functions into libperf.

Keep those non-namespaced names because they will be shared only between
perf and libperf.

Again, these are not exported functions.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Alexey Budankov <alexey.budankov@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Michael Petlan <mpetlan@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20190721112506.12306-61-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/lib/Build

@@ -5,6 +5,7 @@ libperf-y += evsel.o
 libperf-y += evlist.o
 libperf-y += zalloc.o
 libperf-y += xyarray.o
+libperf-y += lib.o
 
 $(OUTPUT)zalloc.o: ../../lib/zalloc.c FORCE
 	$(call rule_mkdir)