Class-ify ptid_t

Class-ify ptid_t

I grew a bit tired of using ptid_get_{lwp,pid,tid} and friends, so I decided to
make it a bit easier to use by making it a proper class.  The fields are now
private, so it's not possible to change a ptid_t field by mistake.

The new methods of ptid_t map to existing functions/practice like this:

  ptid_t (pid, lwp, tid) -> ptid_build (pid, lwp, tid)
  ptid_t (pid) -> pid_to_ptid (pid)
  ptid.is_pid () -> ptid_is_pid (ptid)
  ptid == other -> ptid_equal (ptid, other)
  ptid != other -> !ptid_equal (ptid, other)
  ptid.pid () -> ptid_get_pid (ptid)
  ptid.lwp_p () -> ptid_lwp_p (ptid)
  ptid.lwp () -> ptid_get_lwp (ptid)
  ptid.tid_p () -> ptid_tid_p (ptid)
  ptid.tid () -> ptid_get_tid (ptid)
  ptid.matches (filter) -> ptid_match (ptid, filter)

I've replaced the implementation of the existing functions with calls to
the new methods.  People are encouraged to gradually switch to using the
ptid_t methods instead of the functions (or we can change them all in
one pass eventually).

Also, I'm not sure if it's worth it (because of ptid_t's relatively
small size), but I have made the functions and methods take ptid_t
arguments by const reference instead of by value.

gdb/ChangeLog:

	* common/ptid.h (struct ptid): Change to...
	(class ptid_t): ... this.
	<ptid_t>: New constructors.
	<pid, lwp_p, lwp, tid_p, tid, is_pid, operator==, operator!=,
	matches>: New methods.
	<make_null, make_minus_one>: New static methods.
	<pid>: Rename to...
	<m_pid>: ...this.
	<lwp>: Rename to...
	<m_lwp>: ...this.
	<tid>: Rename to...
	<m_tid>: ...this.
	(ptid_build, ptid_get_pid, ptid_get_lwp, ptid_get_tid, ptid_equal,
	ptid_is_pid, ptid_lwp_p, ptid_tid_p, ptid_match): Take ptid arguments
	as references, move comment to class ptid_t.
	* common/ptid.c (null_ptid, minus_one_ptid): Initialize with
	ptid_t static methods.
	(ptid_build, pid_to_ptid, ptid_get_pid, ptid_get_tid,
	ptid_equal, ptid_is_pid, ptid_lwp_p, ptid_tid_p, ptid_match):
	Take ptid arguments as references, implement using ptid_t methods.
	* unittests/ptid-selftests.c: New file.
	* Makefile.in (SUBDIR_UNITTESTS_SRCS): Add
	unittests/ptid-selftests.c.
	(SUBDIR_UNITTESTS_OBS): Add unittests/ptid-selftests.o.

gdb/gdbserver/ChangeLog:

	* server.c (handle_v_cont): Initialize thread_resume::thread
	with null_ptid.