Make cap_mkdb and services_mkdb file operations sync

Similar fix was done for passwd and group operations in r285050. When a
temporary file is created and then renamed to replace official file there
are no checks to make sure data was written to disk and if a power cycle
happens at this time, system can end up with a 0 length file

Approved by: bapt
MFC after: 1 week
Sponsored by: Netgate
Differential Revision: https://reviews.freebsd.org/D2982