CLEANUP: global: introduce variable pid_bit to avoid shifts with relative_pid

At a number of places, bitmasks are used for process affinity and to map
listeners to processes. Every time 1UL<<(relative_pid-1) is used. Let's
create a "pid_bit" variable corresponding to this value to clean this up.
diff --git a/src/haproxy.c b/src/haproxy.c
index 8993b89..a22ed32 100644
--- a/src/haproxy.c
+++ b/src/haproxy.c
@@ -118,6 +118,7 @@
 static struct list cfg_cfgfiles = LIST_HEAD_INIT(cfg_cfgfiles);
 int  pid;			/* current process id */
 int  relative_pid = 1;		/* process id starting at 1 */
+unsigned long pid_bit = 1;      /* bit corresponding to the process id */
 
 /* global options */
 struct global global = {
@@ -2655,6 +2656,7 @@
 				shut_your_big_mouth_gcc(write(pidfd, pidstr, strlen(pidstr)));
 			}
 			relative_pid++; /* each child will get a different one */
+			pid_bit <<= 1;
 		}
 
 #ifdef USE_CPU_AFFINITY