guile: Add 'history-append!' procedure.

gdb/
2014-02-26  Ludovic Courtès  <ludo@gnu.org>

	* guile/scm-value.c (gdbscm_history_append_x): New function.
	(value_functions): Add it.

gdb/testsuite/
2014-02-26  Ludovic Courtès  <ludo@gnu.org>

	* gdb.guile/scm-value.exp (test_value_in_inferior): Add
	test for 'history-append!'.

gdb/doc/
2014-02-26  Ludovic Courtès  <ludo@gnu.org>

	* gdb/doc/guile.texi (Basic Guile): Document 'history-append!'.