semanage: disconnect to free libsemanage internals

semanage: disconnect to free libsemanage internals

Destroying the handle does not actually disconnect, see [1].
Also free the key on user removal.

[1]: https://github.com/SELinuxProject/selinux/blob/e9072e7d45f4559887d11b518099135cbe564163/libsemanage/src/direct_api.c#L330

Example adduser leak:

    Direct leak of 1008 byte(s) in 14 object(s) allocated from:
        #0 0x5638f2e782ae in __interceptor_malloc (./src/useradd+0xee2ae)
        #1 0x7fb5cfffad09 in dbase_file_init src/database_file.c:170:45

    Direct leak of 392 byte(s) in 7 object(s) allocated from:
        #0 0x5638f2e782ae in __interceptor_malloc (./src/useradd+0xee2ae)
        #1 0x7fb5cfffc929 in dbase_policydb_init src/database_policydb.c:187:27

    Direct leak of 144 byte(s) in 2 object(s) allocated from:
        #0 0x5638f2e782ae in __interceptor_malloc (./src/useradd+0xee2ae)
        #1 0x7fb5cfffb519 in dbase_join_init src/database_join.c:249:28

    [...]