Switch search paths to classic delimited strings

This switches the APIs for setting and getting the global/system
search paths from using git_strarray to using a simple string with
GIT_PATH_LIST_SEPARATOR delimited paths, just as the environment
PATH variable would contain.  This makes it simpler to get and set
the value.

I also added code to expand "$PATH" when setting a new value to
embed the old value of the path.  This means that I no longer
require separate actions to PREPEND to the value.