core: merge the second CapabilityBoundingSet= lines by AND when it is prefixed with...

core: merge the second CapabilityBoundingSet= lines by AND when it is prefixed with tilde (#6724)

If a unit file contains multiple CapabilityBoundingSet= or
AmbientCapabilities= lines, e.g.,
===
CapabilityBoundingSet=CAP_A CAP_B
CapabilityBoundingSet=~CAP_B CAP_C
===
before this commit, it results all capabilities except CAP_C are set to
CapabilityBoundingSet=, as each lines are always merged by OR.
This commit makes lines prefixed with ~ are merged by AND. So, for the
above example only CAP_A is set.
This makes easier to drop capabilities with drop-in config files.