settings: do not clear secrets on Update() without any secrets (bgo #728920)

When a connection is updated by Update() and the new settings contain *no*
secrets, leave the previous secrets untouched. This makes updating connection
parameters much easier. Users (clients) need not to bother with secrets when
they only want adjust a parameter.

Use case:
 - GetSettings()
 - modify the settings
 - Update()

E.g. nmcli con mod my-wifi connection.zone home

https://bugzilla.gnome.org/show_bug.cgi?id=728920
core: add ClearSecrets() D-Bus call

It clears all secrets in a connection.
merge: secret handling enhancements (bgo #728920)

https://bugzilla.gnome.org/show_bug.cgi?id=728920
cli: use g_print/g_printerr rather than printf/fprintf

Use g_print() and g_printerr(), which wrap printf() and
fprintf(stderr,...), but handle conversion from UTF-8 if the locale is
using a different character set.
libnm-core: add nm_utils_wifi_strength_bars(), use it in nmcli and nmtui

Add nm_utils_wifi_strength_bars(), which figures out whether the
terminal can display graphical wifi strength bars, and converts a
numerical value to the appropriate Unicode or ASCII characters.

This also now takes into consideration the fact that the console font
doesn't contain all of the necessary characters, so we can't display
the graphical bars there. (rh #1131491)
tui, cli: improve printing of Wi-Fi strength bars (rh #1131491)
po: update Tamil (ta) translation (bgo #737419)

https://bugzilla.gnome.org/show_bug.cgi?id=737419
po: update Kannada (kn) translation (bgo #737131)

https://bugzilla.gnome.org/show_bug.cgi?id=737131
po: update Telugu (te) translation (bgo #737204)

https://bugzilla.gnome.org/show_bug.cgi?id=737204
po: update Hindi (hi) translation (bgo #737097)

https://bugzilla.gnome.org/show_bug.cgi?id=737097
po: update Marathi (mr) translation (bgo #737057)

https://bugzilla.gnome.org/show_bug.cgi?id=737057
po: update Assamese (as) translation (bgo #736849)

https://bugzilla.gnome.org/show_bug.cgi?id=736849
po: update Swedish (sv) translation (bgo #737086)

https://bugzilla.gnome.org/show_bug.cgi?id=737086
bluez: split out errors

We'll use them from more places than nm nm-bt-device.c in the future.
bluez: track adapter address in NMBluezDevice

We'll need it for bluez5 DUN support.

[lkundrak@v3.sk: Turn the addresses to strings from guint8[ETH_ALEN], as that
is what rest of NetworkManager uses for MAC addresses and what Bluez utility
functions expect as well.]
bluez: implement DUN connect/disconnect for Bluez5

Since Bluez5 likes removing stuff just do what Bluez4 did, and what
the "dund utility in Bluez5 does: hit up the kernel directly, since
the kernel APIs can't ever change.

[lkundrak@v3.sk: one single whitespace fix]
bluez: re-add DUN support for Bluez5
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Cleanups

Clean up most unfinised things, typoes, compile errors, etc. Still won't
compile due to address change from uint8[6] to char *, which is addressed by
the next commit.