[PATCH 1/5] Enhance slurm parser to prepare layouts conf files management

Current slurm parser is mostly dedicated to manage key=value entries in a
configuration file, providing the capability to manage more complex
configuration lines like "key=value attr1=val1 attr2=val2 ..." through the
implementation of custom handlers.

However, custom handlers often had to rewrite their own parsing logic in
order to handle the various subcomponents/attributes.

This commit enhances the parsing logic in order to ease the management of
complex lines using nested parsing options definitions, automatic expansions of
array-like attributes and automatic merges of related lines.

These enhancements are the basis of the layouts configurations parsing logic

(Adapted for slurm-14.11 by Matthieu Hautreux <matthieu.hautreux@cea.fr>)