lib/parse_ini.[ch]: Change code formatting

Change the indentation and formatting of the code in lib/parse_ini.c.
This breaks patches against that file and makes it harder to track its
history, but it (hopefully) improves readability a lot.
lib/parse_ini.c: Fix Clang warnings
lib/parse_ini.c: Cosmetic changes to comments
lib/parse_ini.c: Add comment on NAGIOS_CONFIG_PATH

We might want to spit out a warning when NAGIOS_CONFIG_PATH is used.

While at it, move the function that handles this environment variable to
the bottom.