gdbserver, read_ptid: handle '0' and '-1' thread ids

The special thread id '-1' means 'all threads'.
The special thread id '0' means 'any thread'.

Read_ptid () currently returns

    <current pid>.-1.0

and

    <current pid>.0.0

respectively.

Change that to minus_one_ptid for '-1' and to null_ptid for '0'.

CC: Thiago Jung Bauermann  <thiago.bauermann@linaro.org>
Approved-By: Tom Tromey <tom@tromey.com>