Cleanup: mark utils_get_home_dir as returning a const string

utils_get_home_dir() returns a string obtained from getenv() that
should not be modified nor free'd. However, utils_get_user_home_dir()
returns the path as a copy.

The return parameter of utils_get_home_dir() is marked as const
to underline this difference and prevent mix-ups in the use of
those functions.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>