sysutils/zrepl: Update to 0.2.1

• Update to 0.2.1 [1]
• Unbreak build on aarch64, by updating to newer golang:sys [2]
• Move sample file for syslog.d and newsyslog.conf.d to EXAMPLESDIR [3]
• Migrate to USES=go:modules to simplify building go dependency list
• Add new EXAMPLES option to allow installing these to be optional

Changes this release:

https://github.com/zrepl/zrepl/blob/v0.2.1/docs/changelog.rst

PR: 243354 [1], 242456 [2], 229445 [3]
Submitted by: dmgk [2]
Reported by: asomers [1], yuri [3]