Add two functions to check and change the SIGSEGV behaviour when loading plugins.

Add two functions to check and change the SIGSEGV behaviour when loading plugins.

Original commit message from CVS:
* docs/gst/gstreamer-sections.txt:
* gst/gst.c: (gst_segtrap_is_enabled), (gst_segtrap_set_enabled):
* gst/gst.h:
* gst/gstplugin.c: (_gst_plugin_fault_handler_restore):
Add two functions to check and change the SIGSEGV behaviour
when loading plugins.
Don't mess with the SIGSEGV handler when we were told not to.
Fixes #347794.
API: gst_segtrap_is_enabled
API: gst_segtrap_set_enabled