slave/spawn: restore SIGHUP handler after fork(), before exec()

Foot installs a SIG_IGN handler for SIGHUP. Ignored signals are
inherited in sub-processes. Thus, we need to restore it to SIG_DFL
before exec:ing.

CHANGELOG.md

@@ -151,6 +151,7 @@
   in the right-most column.
 * Multi-column characters being cut in half when resizing the
   alternate screen.
+* Restore `SIGHUP` in spawned processes.
 
 
 ### Security