Better priorities & LOG_UPTO for syslog.h

* Fix PERROR to match musl/glibc better
* More unit tests + enabled on Linux
* Pack priority and facility into one i32 and check the bits with
  bitflags
* Add LOG_UPTO (logic is straight from musl)

Not done:
* "%m" - this could just be added to printf
* LOG_CONS

tests/expected/bins_dynamic/sys_syslog.stderr

@@ -0,0 +1,13 @@
+relibc_test: This is a test message with formatting: 5
+relibc_test: Hank Hill
+relibc_test: Foo has been bar'd
+relibc_test: And now, Eeveelutions
+relibc_test: Espeon
+relibc_test: Umbreon
+relibc_test: Sylveon
+relibc_test: Glaceon
+relibc_test: Leafeon
+relibc_test: Vaporeon
+relibc_test: Flareon
+relibc_test: Jolteon
+relibc_test: Bye from relibc's syslog tests!

tests/expected/bins_static/sys_syslog.stderr

@@ -0,0 +1,13 @@
+relibc_test: This is a test message with formatting: 5
+relibc_test: Hank Hill
+relibc_test: Foo has been bar'd
+relibc_test: And now, Eeveelutions
+relibc_test: Espeon
+relibc_test: Umbreon
+relibc_test: Sylveon
+relibc_test: Glaceon
+relibc_test: Leafeon
+relibc_test: Vaporeon
+relibc_test: Flareon
+relibc_test: Jolteon
+relibc_test: Bye from relibc's syslog tests!