Eliminate name clash with system signal.h header file.

Tried to solve the signal.h name clash by using #include_next, but
seeing as this is not recommended[1] and breaks builds on some cross
compilers, the files were simply renamed: signal.[ch] --> sig.[ch]

[1] - http://gcc.gnu.org/onlinedocs/cpp/Wrapper-Headers.html

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>

plugins/initctl.c

@@ -23,11 +23,11 @@
  */
 
 #include <errno.h>
+#include <signal.h>
 #include <string.h>
 
 #include "finit.h"
 #include "plugin.h"
-#include "signal.h"
 
 #define INIT_MAGIC		0x03091969
 #define INIT_CMD_RUNLVL		1