fileio: modernize get_proc_field()

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
  da65941c3ee03495541c3bffbccc9012c8d9a5f8 for details on file types)