Unset have_argb_visual if !use_argb_visual

This fixes an X error/crash after reloading if the new config sets
own_window_argb_visual to no.
Fix $execbar and $execibar without X

These would fail to show a bar because these variants don't have
special_data set. They should just (like the X11 variant) use the
default_bar_width and fall back on DEFAULT_BAR_WIDTH_NO_X if that is 0.
Check uc->thread before passing it to pthread_join

It's possible to get here before threading started, which causes
pthread_join to segfault. E.g. the following command would CRIT_ERR if
`foo` exists in cwd, and subsequently segfault:
`conky -t '${fs_size foo}'`
Use get_fs_type if path is not absolute

/proc/mounts or /etc/mtab contain only absolute mountpoints, so passing
a relative path would never find anything.
Do not CRIT_ERR in get_fs_type

With commit 6a39a84, the first bit should be unreachable, and the second
bit was unreachable already. Either way, if something strange happens,
this function should not CRIT_ERR, it should just report the fs type as
"unknown".