nspawn: Fix calculation of capabilities for configuration file (#7087)

The current code shifting an integer 1 failed for capabilities like
CAP_MAC_ADMIN (numerical value 33). This caused issues when specifying
them in the nspawn configuration file. Using an uint64_t 1 instead.

The similar code for processing the --capability command line option
was already correctly working.

(cherry picked from commit 1898e5f9)