Avoid calls to strlen in path processing functions.

Avoid calls to strlen in path processing functions.
* fileio.c (file_name_as_directory): Add comment.  Change to add
srclen argument and return the length of result.  Adjust users
accordingly.
(directory_file_name): Fix comment.  Change to add srclen argument,
swap 1nd and 2st arguments to obey the common convention.  Adjust
users accordingly.
* filelock.c (fill_in_lock_file_name): Avoid calls to strlen.