First cut at subset=pid proc mount handling

The procfs mount option subset=pid only shows the processes, not things
such as /proc/stat etc.

For certain programs, this should mean they still work, but have reduced
functionality. This is the first cut at some of them.

pgrep - Removed always loading uptime which we never used anyway. The
program now works fine unless we use --older. Add note in man page
stating it will silently fail.

ps - Load boot time and memory total only when required instead of
always. Changed the error messages to something the user actually
cares about "can't get system boot time" vs "create a structure".
Works for most fields except starts and percent memory.

uptime - Give more useful error messages if uptime not available.

vmstat - move header generation after testing for required proc
files, makes the default output more consistent with the rest
of the options.

References:
 procps-ng/procps#227
 https://www.kernel.org/doc/html/latest/filesystems/proc.html#chapter-4-configuring-procfs
 6814ef2d99

Signed-off-by: Craig Small <csmall@dropbear.xyz>

uptime.c

@@ -44,7 +44,7 @@ static void print_uptime_since()
 
     /* Get the uptime and calculate when that was */
 	if (procps_uptime(&uptime_secs, &idle_secs) < 0)
-		xerrx(EXIT_FAILURE, "uptime");
+		xerr(EXIT_FAILURE, _("Cannot get system uptime"));
     up_since_secs = (time_t) ((now - uptime_secs) + 0.5);
 
     /* Show this */
@@ -72,6 +72,7 @@ static void __attribute__ ((__noreturn__)) usage(FILE * out)
 int main(int argc, char **argv)
 {
     int c, p = 0;
+    char *uptime_str;
 
     static const struct option longopts[] = {
         {"pretty", no_argument, NULL, 'p'},
@@ -110,8 +111,13 @@ int main(int argc, char **argv)
         usage(stderr);
 
     if (p)
-        printf("%s\n", procps_uptime_sprint_short());
+        uptime_str = procps_uptime_sprint_short();
     else
-        printf("%s\n", procps_uptime_sprint());
+        uptime_str = procps_uptime_sprint();
+
+    if (!uptime_str || uptime_str[0] == '\0')
+       xerr(EXIT_FAILURE, _("Cannot get system uptime"));
+
+    printf("%s\n", uptime_str);
     return EXIT_SUCCESS;
 }