lib/parse_ini.[ch]: Simplify code

Rewrite the code that looks up the INI configuration file path (used by
the Extra-Opts feature) in order to improve readability.  The behaviour
should not have changed.
lib/parse_ini.c: Read "monitoring-plugins.ini"

Read "monitoring-plugins.ini" if that file exists, but fall back to
reading "plugins.ini" or "nagios-plugins.ini" for backward
compatibility.
lib/parse_ini.c: Read $MP_CONFIG_FILE

Read $MP_CONFIG_FILE if that variable is set in the environment.
lib/parse_ini.c: Remove outdated comment and code

The lib/parse_ini.c:np_get_defaults() function now dies if no
configuration file is found.
lib/parse_ini.c: Don't cast malloc(3) result

There's no need to cast malloc(3)'s return value.