signal: Add idempotent block function

This makes it trivial to block individual signals from being handled by
a thread. Useful when tryng to set up signal handling in event loops.

Tested:
    Builds and passes unit tests.

Change-Id: I61739debe2a47ec0ec3e767cf138125c6f59165f
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/.gitignore b/.gitignore
index 7d1eea5..205819a 100644
--- a/.gitignore
+++ b/.gitignore
@@ -41,4 +41,4 @@
 /src/stdplus.pc
 
 # Output binaries
-/test/placeholder
+/test/signal