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/Android.bp b/libc/Android.bp
index ffa8222..dc3c73f 100644
--- a/libc/Android.bp
+++ b/libc/Android.bp
@@ -269,7 +269,7 @@
         "-DTHREAD_SAFE=1",
         // The name of the tm_gmtoff field in our struct tm.
         "-DTM_GMTOFF=tm_gmtoff",
-        // TZDEFAULT is not applicable to Android as there is no one file per time zone mapping
+        // Android uses a system property instead of /etc/localtime, so make callers crash.
         "-DTZDEFAULT=NULL",
         // Where we store our tzdata.
         "-DTZDIR=\"/system/usr/share/zoneinfo\"",