Fix tzalloc(nullptr) and add a test.

This works (by reading /etc/localtime) on NetBSD, but not on Android
since we have no such file. Fix that by using our equivalent system
property instead.

Also s/time zone/timezone/ in documentation and comments. We've always
been inconsistent about this (as is upstream in code comments and
documentation) but it seems especially odd now we expose a _type_ that
spells it "timezone" to talk of "time zone" even as we're describing
that type and its associated functions.

Bug: https://github.com/chronotope/chrono/issues/499
Test: treehugger
Change-Id: I142995a3ab4deff1073a0aa9e63ce8eac850b93d
diff --git a/libc/system_properties/context_lookup_benchmark_data.h b/libc/system_properties/context_lookup_benchmark_data.h
index b928875..1d7250c 100644
--- a/libc/system_properties/context_lookup_benchmark_data.h
+++ b/libc/system_properties/context_lookup_benchmark_data.h
@@ -321,7 +321,7 @@
 persist.vendor.         u:object_r:vendor_default_prop:s0
 vendor.                 u:object_r:vendor_default_prop:s0
 
-# Properties that relate to time / time zone detection behavior.
+# Properties that relate to time / timezone detection behavior.
 persist.time.           u:object_r:time_prop:s0
 
 # Properties that relate to server configurable flags