Move pid_running() to process.h

src/common/process.h

@@ -25,6 +25,12 @@
 // pulling in Boost.Process' Boost.Filesystem dependency (which would defeat the
 // entire purpose).
 
+/**
+ * Check whether a process with the given PID is still active (and not a
+ * zombie).
+ */
+bool pid_running(pid_t pid);
+
 /**
  * Helper to create an `environ`-like environment object for passing to the
  * `exec*e()` family of functions.