Fall back on $HOME/.local/share, if necessary. Return "\0" on fail, just
like the WIndows version.

Use system("mkdir -p") to crate the directory structure in
Sys_RegisterQWURLProtocol_f()

Fixes a bug where having an executable called for example
'/path/to/quake/weird\name' (with a backslash in the filename) would lead
to FS_InitFilesystemEx(true) using '/path/to/quake/test' as com_basedir
instead of '/path/to/quake'.

AddUserDirectory should still use $HOME. Also change the windows
Sys_HomeDirectory to return a pointer to a malloc-ed string that
has to be freed so that it watches the posix version.

Fallback to com_gamedir. Also detriplicate code for finding the path of
the config file.
Change Signature for Sys_HomeDirectory since it's no longer const.
Fix some mistakes from previous commits.

"Fix" for functions using com_homedir not checking whether it actually
contains a path, leading to paths that should be relative to com_homedir
actually being absolute.
Also remove the now unnecessary checks that did exist in some functions.