tree-wide: unsetenv cannot fail

... when called with a valid environment variable name. This means that
any time we call it with a fixed string, it is guaranteed to return 0.
(Also when the variable is not present in the environment block.)

src/test/test-execute.c

@@ -898,11 +898,11 @@ int main(int argc, char *argv[]) {
         }
 #endif
 
-        (void) unsetenv("USER");
-        (void) unsetenv("LOGNAME");
-        (void) unsetenv("SHELL");
-        (void) unsetenv("HOME");
-        (void) unsetenv("TMPDIR");
+        assert_se(unsetenv("USER") == 0);
+        assert_se(unsetenv("LOGNAME") == 0);
+        assert_se(unsetenv("SHELL") == 0);
+        assert_se(unsetenv("HOME") == 0);
+        assert_se(unsetenv("TMPDIR") == 0);
 
         can_unshare = have_namespaces();
 

src/test/test-path-util.c

@@ -184,7 +184,7 @@ static void test_find_executable_full(void) {
         if (p)
                 assert_se(oldpath = strdup(p));
 
-        assert_se(unsetenv("PATH") >= 0);
+        assert_se(unsetenv("PATH") == 0);
 
         assert_se(find_executable_full("sh", true, &p) == 0);
         puts(p);
@@ -347,7 +347,7 @@ static void test_fsck_exists(void) {
         log_info("/* %s */", __func__);
 
         /* Ensure we use a sane default for PATH. */
-        unsetenv("PATH");
+        assert_se(unsetenv("PATH") == 0);
 
         /* fsck.minix is provided by util-linux and will probably exist. */
         assert_se(fsck_exists("minix") == 1);

src/test/test-time-util.c

@@ -480,7 +480,7 @@ static void test_in_utc_timezone(void) {
         assert_se(streq(tzname[0], "CET"));
         assert_se(streq(tzname[1], "CEST"));
 
-        assert_se(unsetenv("TZ") >= 0);
+        assert_se(unsetenv("TZ") == 0);
 }
 
 static void test_map_clock_usec(void) {