lldpcli: `configure system interface pattern` instead of `configure lldp iface-pattern`

`configure system` will contain non-LLDP related stuff. Moreover, we
may add `configure system interface alias rewrite` to enable the
rewrite of ifalias (a long awaited feature).
lldpcli: split `conf.c` into `conf-system.c` and `conf-lldp.c`.
lldpcli: `configure system description` to override chassis description
lldpcli: `configure system platform` to override platform description