Introduce catchpoint class

This introduces a catchpoint class that is used as the base class for
all catchpoints.  init_catchpoint is rewritten to be a constructor
instead.

This changes the hierarchy a little -- some catchpoints now inherit
from base_breakpoint whereas previously they did not.  This isn't a
problem, as long as re_set is redefined in catchpoint.

gdb/break-catch-syscall.c

@@ -355,7 +355,6 @@ create_syscall_event_catchpoint (int tempflag, std::vector<int> &&filter)
 
   std::unique_ptr<syscall_catchpoint> c
     (new syscall_catchpoint (gdbarch, std::move (filter)));
-  init_catchpoint (c.get (), gdbarch, tempflag, nullptr);
 
   install_breakpoint (0, std::move (c), 1);
 }