fix SYS_madvise check in flush_one_syscall
Fix typo.
Remove signal dance when launching debugger.

We just need to do a blocking read from the pipe. We don't need to check waitpid.
If the child dies unexpectedly, our pipe read will return EOF as long as we've
closed our write-end of the pipe.
Rename dbg_context to GdbContext.
Move definition of GdbContext from debugger_gdb.cc to debugger_gdb.h and make it a class.
Rename dbg_threadid_t to GdbThreadId.
Rename DbgRegister to GdbRegisterValue.
Rename DbgRegfile to GdbRegisterFile.
Rename DbgRequestType to GdbRequestType.
Rename DbgRestartType to GdbRestartType.
Rename dbg_request to GdbRequest.
Rename dbg_auxv_pair to GdbAuxvPair.
Make ANY and ALL GdbThreadIds members of GdbThreadId.
Switch INTERRUPT_CHAR to a variable instead of a #define.
Remove unnecessary 'static'.
Move DBG_REG_MAX_SIZE to GdbRegisterValue::MAX_SIZE.
Remove symbol_serving_lookups as it's unused.
Make GdbContext::no_ack a bool and give GdbContext a constructor that initializes no_ack.
Remove unnecessary insize and outsize fields.
Remove sockaddr from GdbContext.