MFH: r522323

net-p2p/c-lightning: fix bitcoin-cli exec setup

lightningd does execute bitcoin-cli internally during runtime.

Fix two problems with that:

• The bitcoin-cli executable is usually located in /usr/local/bin/bitcoin-cli but service(8) would remove /usr/local/bin from PATH before executing our rc script /usr/local/etc/rc.d/lightningd and so the lightningd daemon would inherit a PATH that does not contain bitcoin-cli. To fix this give the full path to bitcoin-cli to lightningd.

• bitcoin-cli(1) tries to create its datadir when it starts if it does not exist. By default that is ${HOME}/.bitcoin. service(8) would set HOME=/ and if / is mounted read-only, then this creation would fail. Because we don't want this directory created (it is not necessary and remains empty) tell lightningd to execute bitcoin-cli --datadir=/some/directory/that/already/exists.

Also, append ${lightningd_extra_args} to the lightingd arguments. This was
forgotten in the initial commit and setting lightningd_extra_args would have
had no effect.

Reported by: https://github.com/bitcoin-software

Approved by: portmgr ("fix broken ports")