Tighten security

[manevich]

appimage.c:

• remove redundant mkdir, chown, chmod calls, last two also pose race condition risk.
• open appimage file as user, to avoid race condition, that may alllow attacker mount appimage/iso file that unreadable to user.

firejail.h:

• add ASSERT_PERMS macros

util.c:

• change copy_file function to take uid, gid, and mode for new file, use fchown and fchmod to prevent race condition when setting owner and permissions.

euid_common.h:

• set egid when switching euid, so files created as root won't need additional chown to change group.
• exit with error if unable to switch euid/egid to user - if we continue to run code than intended to run as user with root privileges - this is disaster.

fs.c and other:

• Avoid chown and chmod where possible, using functions and macros mentioned above.

[netblue30]

Merged, thanks.