Some server file fixes

[hgraeber]

This fixes some issues with -f serverfile:

• It makes the -f serverfile option work when the HOST environment variable is set (HOST variable exists in my OS (OpenSUSE) #94). OpenSUSE systems have HOST set per default. There is another pull request (Fix HOST variable not initialized, but inherited from external scope #129), which disables HOST completely. Here the use of a HOST environ variable is treated as feature and is still functional, as long as none of -s serverfile, -c certfile or -d cert directory is used.
• It makes -f serverfile work with process substitution (-f does not work with process substitution #123)
• It allows leading and trailing whitespace in the serverfile (Version 4.14 is not able to trim white spaces from the lines given in a file. #122). As a side effect it will be easier to extend the file with additional columns, if needed.