syslog: local syslog and stdout redirection

[jkellerer]

In relation to #343, this PR adds local syslog server and command output redirection into syslog.

Motivation: Currently I redirect systemd output to syslog (including all stdout/err from resticprofile). This PR allows to have the same support using the built-in syslog client, which has the advantage of more consistent timestamps (not duplicated in stdout formatted log).

The following additional log URLs are added:

• syslog: (local syslog server via unix socket)
• syslog://hostname (udp on default port 514)
• syslog://hostname:514 (udp on port 514)
• syslog-tcp://hostname (tcp on default port 514)
• syslog-tcp://hostname:514 (tcp on port 514)

[jkellerer]

@creativeprojects , do you have any concerns with this. It does change previous behaviour in that it now sends command output by default (though #343 allows to control it).

On the other hand the previous version was a bit inconsistent since we sent it to log files but not to syslog.

[creativeprojects]

Yeah I'm fine with it 👍🏻 Thanks for the PR 😉

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 84.61538% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 70.91%. Comparing base (2a7d5fc) to head (8c12867).

Files	Patch %	Lines
wrapper.go	0.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #344 +/- ## ========================================== + Coverage 70.90% 70.91% +0.01% ========================================== Files 121 121 Lines 12299 12304 +5 ========================================== + Hits 8720 8725 +5 Misses 3185 3185 Partials 394 394 ``` | [Flag](https://app.codecov.io/gh/creativeprojects/resticprofile/pull/344/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Fred) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/creativeprojects/resticprofile/pull/344/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Fred) | `70.91% <84.62%> (+0.01%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Fred#carryforward-flags-in-the-pull-request-comment) to find out more.

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[jkellerer]

Marked it ready for a final review, manually tested all options.