Ignore setsid error in some cases.

If the calling process has already been a leading process of session.
setsid just fail with EPERM, ignore such error.

Test: killall adb;exec 3>f;adb fork-server server --reply-fd 3 & cat f
Change-Id: I1aeac079f29e10aa63ed724b5a43663f25c25ad5
Signed-off-by: Tao Wu <lepton@google.com>