Document gdb.in/gdb.cmd files and debugredirect cli command

Document gdb.in/gdb.cmd files and debugredirect cli command

Add missing documentation for the debugredirect setting.

Add description and uses of gdb.in/gdb.cmd to the testsuite README.

Mention this in the NEWS file.

gdb/ChangeLog:

	* NEWS: Add debugredirect and testsuite sections.

gdb/doc/ChangeLog:

	* gdb.texinfo (Shell Commands): Add debugredirect.

gdb/testsuite/ChangeLog:

	* README (Re-running Tests Outside The Testsuite): New section.