Handle bool specially in gdb.set_parameter

PR python/29217 points out that gdb.parameter will return bool values,
but gdb.set_parameter will not properly accept them.  This patch fixes
the problem by adding a special case to set_parameter.

I looked at a fix involving rewriting set_parameter in C++.  However,
this one is simpler.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29217

gdb/testsuite/gdb.python/py-parameter.exp

@@ -126,6 +126,10 @@ proc_with_prefix test_boolean_parameter { } {
 	"The state of the Test Parameter is off.*" "show parameter off"
     gdb_test "python print (test_param.value)" "False" \
 	"test boolean parameter value is False"
+    gdb_test_no_output "python gdb.set_parameter('print test-param', True)" \
+	"set boolean parameter using set_parameter"
+    gdb_test "python print(gdb.parameter('print test-param'))" "True" \
+	"get boolean parameter using gdb.parameter"
     gdb_test "help show print test-param" \
 	[multi_line \
 	     "Show the state of the boolean test-param" \