nuageinit: fix command injection and related issues

• Add shell_escape() helper to safely escape shell arguments
• Apply shell_escape to all user-controlled values in shell commands: adduser (usershow, useradd, lock, primary_group, groups) addgroup (groupshow, groupadd, members) exec_change_password (usermod) settimezone (tzsetup root and timezone) install_package (pkg package names)
• Escape double quotes in hostname when writing rc.conf.d/hostname
• Add missing 'local' declaration for resolvconf_command in nameservers()
• Escape interface name in resolvconf -a command
• Change open_resolvconf_conf() from 'w' to 'a' mode to prevent data loss when nameservers() is called multiple times
• Clean up stale resolvconf.conf at the start of each boot (skip on postnet to preserve config written by first call)

MFC After: 1 day