Not working pre-create pid-file dir (/run/i2pd).
It fixed with one of this ways:
> PermissionsStartOnly=True
or
> ExecStartPre=/bin/mkdir -p -m 0700 /var/run/i2pd
> ExecStartPre=/bin/chown i2pd: /var/run/i2pd
First way is prefer because RuntimeDirectory's options already used.
https://github.com/PurpleI2P/i2pd/pull/1084#issuecomment-362215861
```
Resolving Dependencies
--> Running transaction check
---> Package i2pd.x86_64 0:2.17.0-20171206git.el7.centos will be updated
---> Package i2pd.x86_64 0:2.18.0-1.el7.centos will be obsoleting
---> Package i2pd-systemd.x86_64 0:2.17.0-20171206git.el7.centos will be obsoleted
--> Finished Dependency Resolution
Dependencies Resolved
==========================================================================
Package Arch Version Repository Size
==========================================================================
Installing:
i2pd x86_64 2.18.0-1.el7.centos vorona-i2pd 915 k
replacing i2pd-systemd.x86_64 2.17.0-20171206git.el7.centos
Transaction Summary
==========================================================================
Install 1 Package
Total download size: 915 k
Is this ok [y/d/N]:
```
@l-n-s thx, obsoletes tag is unneeded in next release)
