Merge pull request #3 from dnozay/variable

Use variables for local/remote paths to zfs binary.
fix typo
add very simple PID file handling to prevent multiple concurrent jobs

Oops, the LOCK file handling was only for preventing running after error --
it did nothing to prevent multiple backup jobs from running at the same time.
Normally not a problem, but a slow resilver on the destination could cause
overlapping jobs.

This is just about the bare minimum, and can/should be improved.
zfssnap user doesn't have write access to /var/run, so use /var/tmp

Perhaps this could be allowed with an ACL or RBAC, or use some std.
SMF PID file handling, but for now just use a dir we're guaranteed
to be able to write to.
Use /usr/bin/env ksh for portability
Allow path to pfexec to be specified as a configuration option

OSes which don't have pfexec (namely FreeBSD) can use sudo instead.
Include verbose switch in debug log message
support for a local mode when REMHOST=localhost

In this mode, ssh is bypassed, and $REMUSER and $REMZFS are ignored.
don't leave PID file when aborting for no sources
abort when no matching local snapshots found
Merge branch 'pvalkone-freebsd-portability-fixes'

Closes #4.
Resolved Conflicts:
	zfs-backup.sh
add a makefile and a cron job.

inspired from the `zfs-auto-backup` Makefile,
- we can install the `zfs-backup` script in `/sbin`,
- create cron jobs,
- create default configuration file.