Syscall DPRINTF and warning cleanup.

Syscall DPRINTF and warning cleanup.

base/trace.hh:
    Need std:: on DPRINTFR reference to string class.
base/traceflags.py:
    Remove SyscallWarnings trace flag... we should always print warnings
    so nothing undesirable goes unnoticed.  Replaced with (currently unused)
    Syscall flag.
sim/syscall_emul.cc:
    Change SyscallWarning DPRINTFs into warn() calls.
    Uncomment SyscallVerbose DPRINTFs.
sim/syscall_emul.hh:
    Change SyscallWarning DPRINTFs into warn() calls.
    Call fatal() instead of ad-hoc termination.

--HG--
extra : convert_revision : dc6c2ce3691a129f697b6a6ae5d889e2dbaab228