tree-wide: make use of getpid_cached() wherever we can

This moves pretty much all uses of getpid() over to getpid_raw(). I
didn't specifically check whether the optimization is worth it for each
replacement, but in order to keep things simple and systematic I
switched over everything at once.

src/basic/fd-util.c

@@ -31,6 +31,7 @@
 #include "missing.h"
 #include "parse-util.h"
 #include "path-util.h"
+#include "process-util.h"
 #include "socket-util.h"
 #include "stdio-util.h"
 #include "util.h"
@@ -282,7 +283,7 @@ int same_fd(int a, int b) {
                 return true;
 
         /* Try to use kcmp() if we have it. */
-        pid = getpid();
+        pid = getpid_cached();
         r = kcmp(pid, pid, KCMP_FILE, a, b);
         if (r == 0)
                 return true;