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>

vmstat.c

@@ -373,7 +373,6 @@ static void new_format(void)
 
     sleep_half = (sleep_time / 2);
     hz = procps_hertz_get();
-    new_header();
 
     if (procps_vmstat_new(&vm_info) < 0)
         xerrx(EXIT_FAILURE, _("Unable to create vmstat structure"));
@@ -381,6 +380,7 @@ static void new_format(void)
         xerrx(EXIT_FAILURE, _("Unable to create system stat structure"));
     if (procps_meminfo_new(&mem_info) < 0)
         xerrx(EXIT_FAILURE, _("Unable to create meminfo structure"));
+    new_header();
 
     pgpgin[tog] = VMSTAT_GET(vm_info, VMSTAT_PGPGIN, ul_int);
     pgpgout[tog] = VMSTAT_GET(vm_info, VMSTAT_PGPGOUT, ul_int);