Make get_random_string() return a gchar*.
This is part of the effort to migrate string declarations and
allocations from char to gchar, replacing standard C allocators
(malloc/calloc/strdup) with their glib equivalents
(g_malloc/g_new0/g_strdup).
The primary goal is to prevent mismatched allocator bugs. While char and
gchar are binary compatible, mixing their respective deallocators is
dangerous:
* Freeing malloc'd memory with g_free() (or vice versa) can lead to
memory corruption, double-frees, or crashes.
* This is seems to be the case especiall on non Linux platforms or when
using hardened memory allocators where the GLib slice allocator or
wrappers may differ from the system heap.
By standardizing on gchar, we ensure that our automatic cleanup macros
(like auto_gchar) always invoke the correct deallocator (g_free).
Fixes https://github.com/profanity-im/profanity/issues/1794
Explanation
The problem is the download's identifier. Downloads are given an ID so they can be referenced later when their progress changes. Currently, the download's ID is the download's URL. When you download the same file twice, you have two downloads with the same ID. Download progress updates are shown on the first of both downloads with the same ID.
Solution
Change the download's ID from its URL to a random number. A random ID is generated when get_random_string() is called from cmd_funcs.c. Several other functions are updated to cope with the new ID format.
