linux-procfs: Introduce enum proc_state

linux-procfs: Introduce enum proc_state

Parse the process's /proc/PID/status state into an enum instead of the
current scheme of passing state strings around.

gdb/ChangeLog:
2016-07-25  Pedro Alves  <palves@redhat.com>

	* nat/linux-procfs.c (enum proc_state): New enum.
	(parse_proc_status_state): New function.
	(linux_proc_pid_get_state): Replace output string buffer parameter
	with an output proc_state parameter.  Use parse_proc_status_state.
	(linux_proc_pid_is_gone): Adjust to use proc_state values.
	(linux_proc_pid_has_state): Change type of 'state' parameter; now
	an enum proc_state.  Adjust to linux_proc_pid_get_state interface
	change.
	(linux_proc_pid_is_stopped)
	(linux_proc_pid_is_trace_stopped_nowarn)
	(linux_proc_pid_is_zombie_maybe_warn): Adjust to
	linux_proc_pid_get_state interface change.