build-sys: remove commented-out m4 from user@.service

Otherwise this actually remains in the generated unit in /usr/lib.

If you want to keep it commented out, a m4-compatible way would be:

    m4_ifdef(`HAVE_SMACK',
    dnl Capabilities=cap_mac_admin=i
    dnl SecureBits=keep-caps
    )
delta: fix output alignment of [REDIRECTED] entries
blkid: Warn when rejecting a superblock with a bad csum

Bump libblkid requirement from 2.20 to 2.24.
util-linux 2.25 is actually required since fdbbad981cc5da8bb4ed7e9b6646e7a114745ec5