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/process-util.c

@@ -870,7 +870,7 @@ int get_process_umask(pid_t pid, mode_t *ret) {
 
         p = procfs_file_alloca(pid, "status");
 
-        r = get_proc_field(p, "Umask", WHITESPACE, &m);
+        r = get_proc_field(p, "Umask", &m);
         if (r == -ENOENT)
                 return -ESRCH;
         if (r < 0)
@@ -2079,9 +2079,9 @@ int get_process_threads(pid_t pid) {
 
         p = procfs_file_alloca(pid, "status");
 
-        r = get_proc_field(p, "Threads", WHITESPACE, &t);
+        r = get_proc_field(p, "Threads", &t);
         if (r == -ENOENT)
-                return proc_mounted() == 0 ? -ENOSYS : -ESRCH;
+                return -ESRCH;
         if (r < 0)
                 return r;