Simplify DHCP lease file parer after switch to dhcpleased

Simplify DHCP lease file parer after switch to dhcpleased

dhcpleased(8)'s lease files are much simpler than dhclient.leases(5):
- exactly one lease per file (not many)
- only option lines (not statement, declaration, etc.)
- every option/value is separated by a single ": " (no optional leading
  "option ")
- values are not quoted or escaped (like dhclient does)

lease_value() is hard to read, mostly because it strips optional double
quotes around option values as was required with dhclient.

dhclient VIS_ALL's values if they contain backticks, backslashes and
other characters, otherwise it VIS_SAFE's (and optionally quotes them).

dhcpleased VIS_SAFE's all value strings equally.

All install.sub users of lease_value() quote its output so they should
be safe without any special unescaping as previously done.

OK florian