coreutils "unix" feature

[sunfishcode]

Most of coreutils with default features is now supported, except for custom signal handlers. Enabling the "unix" feature evokes undefined reference errors for the following symbols:

These can probably be implemented fairly straightforwardly with current rustix features:

• `cfgetospeed'
• `mknod'
• `uname'
• `getpriority'
• `setpriority'
• `perror'
• `ttyname'
• `ttyname_r'
• `getlogin'

These probably need more work:

• `getgrouplist'
• `getloadavg'
• `getutxent'
• `endutxent'
• `setutxent'
• `utmpxname'
• `sethostname'