supplicant: fix handling of 'freq_list' option and band locking (bgo #737795)

The supplicant has a custom parsing function for freq_list which
handles the list as a string.  Having NM marshal the option
as TYPE_BYTES causes the supplicant to interpret the values that
NM passes (which are in ASCII) as a byte-array and thus the
supplicant gets a bogus frequency list.  Instead, NM should
marshal freq_list as a simple string (using TYPE_KEYWORD without
value checking).

https://bugzilla.gnome.org/show_bug.cgi?id=737795

src/supplicant-manager/nm-supplicant-settings-verify.c

@@ -137,7 +137,7 @@ static const struct Opt opt_table[] = {
 	{ "proactive_key_caching", TYPE_INT,  0, 1, FALSE,  NULL },
 	{ "bgscan",             TYPE_BYTES,   0, 0, FALSE,  NULL },
 	{ "pac_file",           TYPE_BYTES,   0, 1024, FALSE,  NULL },
-	{ "freq_list",          TYPE_BYTES,   0, 0, FALSE,  NULL },
+	{ "freq_list",          TYPE_KEYWORD, 0, 0, FALSE,  NULL },
 };