MEDIUM: tree-wide: logsrv struct becomes logger

MEDIUM: tree-wide: logsrv struct becomes logger

When 'log' directive was implemented, the internal representation was
named 'struct logsrv', because the 'log' directive would directly point
to the log target, which used to be a (UDP) log server exclusively at
that time, hence the name.

But things have become more complex, since today 'log' directive can point
to ring targets (implicit, or named) for example.

Indeed, a 'log' directive does no longer reference the "final" server to
which the log will be sent, but instead it describes which log API and
parameters to use for transporting the log messages to the proper log
destination.

So now the term 'logsrv' is rather confusing and prevents us from
introducing a new level of abstraction because they would be mixed
with logsrv.

So in order to better designate this 'log' directive, and make it more
generic, we chose the word 'logger' which now replaces logsrv everywhere
it was used in the code (including related comments).

This is internal rewording, so no functional change should be expected
on user-side.