gprofng: fix data race

In our GUI project (https://savannah.gnu.org/projects/gprofng-gui), we use
the output of gprofng to display the data. Sometimes this data is corrupted.

gprofng/ChangeLog
2023-06-29  Vladimir Mezentsev  <vladimir.mezentsev@oracle.com>

	* src/ipc.cc (ipc_doWork): Fix data race.
	* src/ipcio.cc (IPCresponse::print): Fix data race.
	Remove unused variables and functions.
	* src/ipcio.h: Declare two variables.
	* src/StringBuilder.cc (StringBuilder::write): New function.
	* src/StringBuilder.h: Likewise.