Patch some memory leaks.

Patch some memory leaks.

capture_dev_user_descr_find() and capture_dev_user_cfilter_find() return
g_malloc()ated strings; we don't need to g_strdup() them to get a
g_malloc()ated string, and we *do* need to free them if we aren't going
to use them any more.

Document that while we're at it, and give more details for the functions
returning integer user-specified interface parameters as well.

Change-Id: Icf98a48992b1c4168ead54bdc4cc2847da89d665
Reviewed-on: https://code.wireshark.org/review/7745
Reviewed-by: Guy Harris <guy@alum.mit.edu>