perf trace: Allow specifying a set of events to add in perfconfig

To add augmented_raw_syscalls to the events speficied by the user, or be
the only one if no events were specified by the user, one can add this
to perfconfig:

  # cat ~/.perfconfig
  [trace]
	  add_events = /home/acme/git/perf/tools/perf/examples/bpf/augmented_raw_syscalls.o
  #

I.e. pre-compile the augmented_raw_syscalls.c BPF program and make it
always load, this way:

  # perf trace -e open* cat /etc/passwd > /dev/null
     0.000 ( 0.013 ms): cat/31557 openat(dfd: CWD, filename: /etc/ld.so.cache, flags: CLOEXEC) = 3
     0.035 ( 0.007 ms): cat/31557 openat(dfd: CWD, filename: /lib64/libc.so.6, flags: CLOEXEC) = 3
     0.353 ( 0.009 ms): cat/31557 openat(dfd: CWD, filename: /usr/lib/locale/locale-archive, flags: CLOEXEC) = 3
     0.424 ( 0.006 ms): cat/31557 openat(dfd: CWD, filename: /etc/passwd) = 3
  #

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Luis Cláudio Gonçalves <lclaudio@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: https://lkml.kernel.org/n/tip-0lgj7vh64hg3ce44gsmvj7ud@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>