[ALL] Check Gearman SSL file access

When using Gearman+SSL, processes can silently and confusingly
fail to connect if the user that owns the process cannot read
the SSL files specified in the configuration file. The private
key file is usually the problem since it is only readable by the
file owner. This adds a check at startup to make sure that the
process owner can read these files, and that they exist.

Change-Id: Ida147f7a6138cacf9e23e797914321d919646a29