fileio: make get_status_field() more generic

fileio: make get_status_field() more generic

All users of get_status_field() expect the field pattern to occur in
the beginning of a line, and the delimiter is ':'.

Hardcode this into the function, and also skip any whitespace before ':'
to support fields in files like /proc/cpuinfo. Add support for returning
the full field value (currently stops on first whitespace).

Rename the function so it's easier to ensure all callers switch to new
semantics.