luci-app-strongswan-swanctl: Encryption proposal disappears when adding tunnel configuration

[mikma]

Steps to reproduce:

1. go to: VPN → strongSwan Configuration → Encryption Proposal
2. enter "vpn1" and click on "Add".
3. select "ESP Proposal".
4. Save & Apply
5. go to: Tunnel Configuration tab
6. enter "vpn1" and click on "Add"
7. select the crypto proposal "vpn1". Enter any local and remote subnet.
8. Save & Apply
9. go to: Encryption Proposal tab again.

Actual behavior:

1. The "vpn1" line in Encryption Proposal is gone.

Expected behavior:

1. The "vpn1" line in Encryption Proposal should exist.

Additional Information:

OpenWrt version information from system /etc/openwrt_release

DISTRIB_ID='OpenWrt'
DISTRIB_RELEASE='SNAPSHOT'
DISTRIB_REVISION='r26515-6e51b363e9'
DISTRIB_TARGET='x86/64'
DISTRIB_ARCH='x86_64'
DISTRIB_DESCRIPTION='OpenWrt SNAPSHOT r26515-6e51b363e9'
DISTRIB_TAINTS=''

This problem is related to Named uci sections may be overwritten by misstake in luci

[systemcrash]

@jempatel could you take a look at this? I've verified this behaviour.

Needs an error check to prevent in-use names from being consumed by a new definition, for each of the section types in use.