Add Guile frame-read-register command

Add Guile frame-read-register command

gdb/ChangeLog:

	* guile/scm-frame.c (gdbscm_frame_read_register): New function.
	(frame_functions): Bind gdbscm_frame_read_register to
	frame-read-register.
	* guile/lib/gdb.scm (frame-read-register): Export.

gdb/doc/ChangeLog:

	* guile.texi (Frames In Guile): Describe frame-read-register.

gdb/testsuite/ChangeLog:

	* gdb.guile/scm-frame.exp: Add frame-read-register tests, modelled
	after the Python tests.