FreeBSD support

[alxwr]

• Mainly path and name adjustments
• Added a hint for running inside FreeBSD jail.

Tested on

• FreeBSD 11.2
• Ubuntu 18.04

[aboe76]

@alxwr what is this postgresql-cluster-prepared ?

[alxwr]

@aboe76 Some distributions do the minimal Postgres setup themselves. Some don't. postgresql-cluster-prepared sets up the DB cluster, (even if it contains only the local DB server).
https://github.com/saltstack-formulas/postgres-formula/blob/master/postgres/server/init.sls#L61

I.e. FreeBSD: initially /usr/local/pgsql is empty.
After running the state it looks like this:

# ls /usr/local/pgsql/data/
PG_VERSION            pg_hba.conf.bak       pg_snapshots/         postgresql.auto.conf
base/                 pg_ident.conf         pg_stat/              postgresql.conf
global/               pg_logical/           pg_stat_tmp/          postmaster.opts
pg_clog/              pg_multixact/         pg_subtrans/          postmaster.pid
pg_commit_ts/         pg_notify/            pg_tblspc/
pg_dynshmem/          pg_replslot/          pg_twophase/
pg_hba.conf           pg_serial/            pg_xlog/

[alxwr]

@aboe76 Thanks for the merge!