PR gdb/22046: Fix T-stopped detach regression on old Linux kernels

PR gdb/22046: Fix T-stopped detach regression on old Linux kernels

On <=RHEL6 hosts Fedora/RHEL GDB started to 'kill -STOP' all processes it
detached.  Even those not originally T-stopped.  This is a Fedora-specific
patch which is based on upstream GDB's PROC_STATE_STOPPED state.

I believe (I did not verify) this patch did regress it:
commit d617208bb06bd461b52ce041d89f7127e3044762
Author: Pedro Alves <palves@redhat.com>
Date:   Mon Jul 25 12:42:17 2016 +0100
    linux-procfs: Introduce enum proc_state

As originally there was strstr() but now there is strcmp() and so the missing
trailing '\n' no longer matches.

The Bug was found by Michal Kolar.

Reproducibility:
$ gdb -p $PID
(gdb) quit
$ ...

Actual results:
===
RHEL6.9 x86_64 # scl enable devtoolset-7 bash
RHEL6.9 x86_64 # which gdb
/opt/rh/devtoolset-7/root/usr/bin/gdb
RHEL6.9 x86_64 # ./testcase.sh
24737 pts/0    S+     0:00 /bin/sleep 4
24737 pts/0    T+     0:00 /bin/sleep 4
RHEL6.9 x86_64 #
===

Expected results:
===
RHEL6.9 x86_64 # which gdb
/usr/bin/gdb
RHEL6.9 x86_64 # ./testcase.sh
24708 pts/0    S+     0:00 /bin/sleep 4
24708 pts/0    S+     0:00 /bin/sleep 4
./testcase.sh: line 20: kill: (24708) - No such process
RHEL6.9 x86_64 #
===

gdb/ChangeLog
2017-09-01  Jan Kratochvil  <jan.kratochvil@redhat.com>

	PR gdb/22046
	* nat/linux-procfs.c (parse_proc_status_state): Fix PROC_STATE_STOPPED
	detection.