library: expanded to provide for the UID used at login

This patch represents the newlib implementation of Jan
Rybar's merge request referenced below. It essentially
moves that code out of the ps program and into our new
library where it's available via the <pids> interface.

Reference(s):
https://gitlab.com/procps-ng/procps/merge_requests/57
https://bugzilla.redhat.com/show_bug.cgi?id=1518986

Signed-off-by: Jim Warner <james.warner@comcast.net>

proc/readproc.h

@@ -165,6 +165,8 @@ typedef struct proc_t {
         *sd_uunit;      // n/a             systemd user unit id
     char
         *lxcname;       // n/a             lxc container name
+    int
+        luid;           // loginuid        user id at login
 } proc_t;
 
 // PROCTAB: data structure holding the persistent information readproc needs
@@ -223,6 +225,7 @@ typedef struct PROCTAB {
 #define PROC_FILLNS          0x8000 // fill in proc_t namespace information
 #define PROC_FILLSYSTEMD    0x80000 // fill in proc_t systemd information
 #define PROC_FILL_LXC      0x800000 // fill in proc_t lxcname, if possible
+#define PROC_FILL_LUID     0x400000 // fill in proc_t luid (login user id)
 
 #define PROC_LOOSE_TASKS     0x2000 // treat threads as if they were processes