fileio: modernize get_proc_field()

- Drop effectively unused "terminator" param, imply whitespace
- Make ret param optional
- Return ENODATA if the requested key is not found, rather than
  ENOENT
- Turn ENOENT -> ENOSYS if /proc/ is not mounted
- Don't skip whitespaces before ':', nothing needs this handling
  anyways
- Remove the special treatment for all "0"s. We don't actually
  use this for capabilities given pidref_get_capability() exists
- Switch away from read_full_virtual_file() - files using "field"
  scheme under /proc/ seem all to be "seq_file"s (refer to
  da65941c3e for details on file types)

src/basic/virt.c

@@ -432,8 +432,8 @@ static Virtualization detect_vm_zvm(void) {
         _cleanup_free_ char *t = NULL;
         int r;
 
-        r = get_proc_field("/proc/sysinfo", "VM00 Control Program", WHITESPACE, &t);
-        if (r == -ENOENT)
+        r = get_proc_field("/proc/sysinfo", "VM00 Control Program", &t);
+        if (IN_SET(r, -ENOENT, -ENODATA))
                 return VIRTUALIZATION_NONE;
         if (r < 0)
                 return r;
@@ -655,7 +655,7 @@ Virtualization detect_container(void) {
         /* proot doesn't use PID namespacing, so we can just check if we have a matching tracer for this
          * invocation without worrying about it being elsewhere.
          */
-        r = get_proc_field("/proc/self/status", "TracerPid", WHITESPACE, &p);
+        r = get_proc_field("/proc/self/status", "TracerPid", &p);
         if (r < 0)
                 log_debug_errno(r, "Failed to read our own trace PID, ignoring: %m");
         else if (!streq(p, "0")) {