Make _posix usage match 'the rules'

Also fix log_posix to get things compiling.
diff --git a/src/core/support/log_posix.c b/src/core/support/log_posix.c
index 1292c9e..ab2d2e5 100644
--- a/src/core/support/log_posix.c
+++ b/src/core/support/log_posix.c
@@ -64,7 +64,7 @@
   va_end(args);
   if (ret < 0) {
     message = NULL;
-  } else if (ret <= sizeof(buf) - 1) {
+  } else if ((size_t)ret <= sizeof(buf) - 1) {
     message = buf;
   } else {
     message = allocated = gpr_malloc(ret + 1);