feat: look up username/groupname for socket ownership

[s3lph]

At the moment, the uid/gid for the ydotoold socket ownership must be provided numerically.

This PR extends the socket owner string parsing with the following logic:

1. Try to parse uid and gid as int
   • If the entire string is consumed (i.e. it was fully numerical), treat it as uid/gid => done
2. Otherwise, treat it as a username or groupname and look it up via getpwnam/getgrnam
   • If the name is not found, exit
   • Otherwise use the found user's uid / group's gid