Rename path_list to string_list

Rename path_list to string_list

The name path_list was correct for the first usage of that data structure,
but it really is a general-purpose string list.

$ perl -i -pe 's/path-list/string-list/g' $(git grep -l path-list)
$ perl -i -pe 's/path_list/string_list/g' $(git grep -l path_list)
$ git mv path-list.h string-list.h
$ git mv path-list.c string-list.c
$ perl -i -pe 's/has_path/has_string/g' $(git grep -l has_path)
$ perl -i -pe 's/path/string/g' string-list.[ch]
$ git mv Documentation/technical/api-path-list.txt \
	Documentation/technical/api-string-list.txt
$ perl -i -pe 's/strdup_paths/strdup_strings/g' $(git grep -l strdup_paths)

... and then fix all users of string-list to access the member "string"
instead of "path".

Documentation/technical/api-string-list.txt needed some rewrapping, too.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>