`/etc/passwd` generation support

[Trung Nguyen]

Add support for HyClone generating /etc/passwd and /etc/groups.

HyClone should:

• Read the host's user accounts. These accounts should be mapped to HyClone UIDs starting from 2000, except for that of the user who started hyclone_server. User ids smaller than 1000 will have the host- prefixed.
• The user who started hyclone_server will be mapped as HyClone UID 0 (the root).
• For other existing users listed in HyClone's /etc/passwd:
  • UIDs starting from 2000 but not listed in the host's user accounts are understood as dangling accounts and should be removed.
  • Other users not listed in the host's user accounts will be mapped to the user who started hyclone_server. In HyClone they should keep their respective ids.

A similar strategy should be applied to group ids.

Also, hyclone_server should allow querying home directories according to the settings in /etc/passwd, to resolve this comment by @X547.

This should allow an actual _kern_setgroups and other similar syscalls in monika. Rather than failing with EPERM 99% of the time and just pretend that it never happened, these syscalls can succeed for HyClone emulated UID/GIDs and fail when the hyclone_server launching user is not the host's root and the binary attempts to request host UID/GIDs.