pgAgent commit: Using mutex around the logging functionality to avoidl

Using mutex around the logging functionality to avoid logging at the
same time from different threads in the same file, and make a mess of
the log file.

Branch
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master

Details
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https://git.postgresql.org/gitweb?p=pgagent.git;a=commitdiff;h=5706a7738f566686ab8a5046572e4c67dccfcc4a

Modified Files
--------------
connection.cpp | 35 -----------------------------------
include/misc.h | 35 +++++++++++++++++++++++++++++++++++
unix.cpp       | 11 +++++++++++
3 files changed, 46 insertions(+), 35 deletions(-)