use changesignal to restore SIGCHLD and SIGINT handling

after GAP intialization, since we don't want it clobbering our own handlers for those
signals, or setting new ones

Something still needs to be done to replace the default SIGCHLD handler; see discussion at
gap-system/gap#3072 (comment)

src/sage/libs/gap/util.pyx

@@ -15,10 +15,12 @@ Utility functions for libGAP
 from __future__ import print_function, absolute_import
 
 import os
+import signal
 import warnings
 
 from cpython.exc cimport PyErr_SetObject
 from cpython.object cimport Py_LT, Py_LE, Py_EQ, Py_NE, Py_GT, Py_GE
+from cysignals.pysignals import changesignal
 from cysignals.signals cimport sig_on, sig_off, sig_error
 
 from .gap_includes cimport *
@@ -269,7 +271,13 @@ cdef initialize():
     # Initialize GAP and capture any error messages
     # The initialization just prints error and does not use the error handler
     try:
-        GAP_Initialize(argc, argv, environ, &gasman_callback, &error_handler)
+        with changesignal(signal.SIGCHLD, signal.SIG_DFL), \
+                changesignal(signal.SIGINT, signal.SIG_DFL):
+            # Need to save/restore current SIGINT handling since GAP_Initialize
+            # currently clobbers it; it doesn't matter what we set SIGINT to
+            # temporarily.
+            GAP_Initialize(argc, argv, environ, &gasman_callback,
+                           &error_handler)
     except RuntimeError as msg:
         raise RuntimeError('libGAP initialization failed\n' + msg)