syslog.h: implement LOG_PERROR.

This is the one openlog() flag that toybox uses. We should probably try
to unify toybox's POSIX logger and Android-specific log at some point,
and this will help.

Also fix our behavior with an empty format string, noticed while adding
tests.

Test: treehugger
Test: adb shell logger -s foo
Change-Id: Ic027e78a460be3db83cc4c6f9946c9efa22be6e1
diff --git a/tests/Android.bp b/tests/Android.bp
index 1be1ec3..281e29d 100644
--- a/tests/Android.bp
+++ b/tests/Android.bp
@@ -503,6 +503,7 @@
         "sys_vfs_test.cpp",
         "sys_wait_test.cpp",
         "sys_xattr_test.cpp",
+        "syslog_test.cpp",
         "system_properties_test.cpp",
         "system_properties_test2.cpp",
         "termios_test.cpp",