Minor modification to Inivar

[havardAasen]

Some minor refactoring in inivar, most is related to Clang-Tidy and Clang warnings.

• Renamed a couple of variables.
• Be more consistent regarding return value, -1 on error and 1 if variable is not found, 0 is still success.
• Change to headers adapted for C++.
• Simplified conditions for several of the if-statements.
• Removed two unused headers.
• Use strtol() instead of sscanf() to get number from command-line argument. This also sets a limit of accepted numbers between 0 and 'INT_MAX'.
• Change name for '-num' argument to 'occurrence_number'.

The return value is the only change noticeable from the outside.

The man page was extended and rewritten in asciidoc.

[havardAasen]

Rebased against master to ensure successful CI integration.