man: fix `make uninstall` to remove the nmtui manual pages

Fixes: 1e8b681d4f10c2b5e9478ec4972cad64aa1f8e7c
Signed-off-by: Thomas Haller <thaller@redhat.com>
tui: fix gateway editing

Since adding NMSettingIPConfig:gateway, we were just binding that
property to the Gateway entry as a string. But this caused two
different problems: first, we were trying to set the :gateway property
from the entry even when the IP address in the entry was incomplete
(causing warnings), and second, we were no longer enforcing the rule
that the gateway can only be set when there are static addresses
configured.

Fix this by adding back nm_editor_bind_ip_gateway_to_string(), but
with new semantics reflecting the new way NMSettingIPConfig:addresses
and :gateway work. (Besides just fixing the new bugs, this also makes
the Gateway entry insensitive when there are no addresses; before,
nmtui would allow you to type there, but the value would not be
saved.)

Fixes: Test263_nmtui_ipv4_addresses_delete_ip_and_back_to_auto

https://bugzilla.gnome.org/show_bug.cgi?id=740017
build: fix configure formatting of IN6_ADDR_GEN_MODE check

Print the result, and make the m4 formatting consistent with the
other kernel checks.
tui: fix alignment of pop-up menus

Pop-up menus were slightly mis-aligned in the main window, and even
more mis-aligned in slave-editing windows. Fix that.

Also add a bit of padding to the pop-up window, because it just looks
better that way.