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/src/Makefile.am b/src/Makefile.am
index de5de8d..5e935f4 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -4,5 +4,5 @@
 libstdplus_la_SOURCES =
 libstdplus_la_LIBADD = $(COMMON_LIBS)
 
-nobase_include_HEADERS += stdplus/placeholder.hpp
-libstdplus_la_SOURCES += stdplus/placeholder.cpp
+nobase_include_HEADERS += stdplus/signal.hpp
+libstdplus_la_SOURCES += stdplus/signal.cpp