fix uid/gid > 2^31

This MR revisits a partial fix from 2018. The problem stems from incorrect
handling of unsigned 32-bit uid_ts and gid_ts as signed when values are
large - i.e. when the high bit is set. In that case, pgrep and pkill fail to
identify processes by uid. (They succeed when finding the same processes by
username.) The primary fix for this is to impliment the "FIXME" comment in
proc/readproc.h, the implementation of which allows the removal of the (int)
casts from the partial fix from 2018.

The other fixed code in this MR consists of tests in strict_atol() that
detects and errors out on overflows.

References:
 Merge !146

proc/readproc.h

@@ -152,12 +152,13 @@ typedef struct proc_t {
         session,        // stat            session id
         nlwp,           // stat,status     number of threads, or 0 if no clue
         tgid,           // (special)       thread group ID, the POSIX PID (see also: tid)
-        tty,            // stat            full device number of controlling terminal
+        tty;            // stat            full device number of controlling terminal
         /* FIXME: int uids & gids should be uid_t or gid_t from pwd.h */
-        euid, egid,     // stat(),status   effective
-        ruid, rgid,     // status          real
-        suid, sgid,     // status          saved
-        fuid, fgid,     // status          fs (used for file access only)
+        uid_t euid; gid_t egid; // stat(),status effective
+        uid_t ruid; gid_t rgid; // status        real
+        uid_t suid; gid_t sgid; // status        saved
+        uid_t fuid; gid_t fgid; // status        fs (used for file access only)
+    int
         tpgid,          // stat            terminal process group id
         exit_signal,    // stat            might not be SIGCHLD
         processor;      // stat            current (or most recent?) CPU