Merge duplicate subsections in profile library

Merge duplicate subsections in profile library

Modify profile_add_node() to return the existing node, rather than
making a new one, when adding subsection configuration.

This fixes an issue where the first instance of a subsection will hide
the second instance entirely.  In particular, it was previously
impossible to split realm-specific configuration across multiple
config files.

[ghudson@mit.edu: adjusted style, added test case]

ticket: 7863
tags: pullup
target_version: 1.16-next
target_version: 1.15-next