Escape backslash in windows path

Hi,
On windows host, we see the following ERROR,

(gdb) PASS: gdb.base/setshow.exp: set history filename ~/foobar.baz
ERROR OCCURED: couldn't compile regular expression pattern: invalid escape \ seq
uence
    while executing
"expect -nobrace -i exp13 -timeout 10 -re {.*A problem internal to GDB has been
detected} {
	    fail "$message (GDB internal error)"
	    gdb_internal..."
    invoked from within
"expect {
-i exp13 -timeout 10
	-re ".*A problem internal to GDB has been detected" {
	    fail "$message (GDB internal error)"
	    gdb_internal_erro..."
    ("uplevel" body line 1)
    invoked from within
"uplevel $body" REGEXP REG_EESCAPE {invalid escape \ sequence} couldn't compile
regular expression pattern: invalid escape \ sequenceERROR: Process no longer ex
ists

which leads to
UNRESOLVED: gdb.base/setshow.exp: show history filename (~/foobar.baz)

and this error is thrown from this test below:

gdb_test "show history filename" \
    "The filename in which to record the command history is \"$HOME/foobar.baz\"..*" \
    "show history filename (~/foobar.baz)"

HOME is a windows path, like C:\foo\bar.  When it is used in gdb_test to match
output, the error is thrown because backslash is a special character in
regular expression.  This patch is to escape backslash to fix this
error by using string_to_regexp.

gdb/testsuite:

2014-04-03  Yao Qi  <yao@codesourcery.com>

	* gdb.base/setshow.exp: Invoke string_to_regexp to HOME and PWD.