Atoms: Avoid allocations in XkbAtomText()

Atoms: Avoid allocations in XkbAtomText()

XkbAtomGetString() returns a freshly-allocated string, whereas
XkbAtomText() returns the same in a temporary buffer.  XkbAtomText used
to call XkbAtomGetString() and then free the result, which seems quite
spectacularly pointless when you think about it.  Shuffle the atom code
around so we don't have to allocate for XkbAtomText().

This changes semantics slightly wrt non-printable characters, but I
haven't been able to see any effect so far.  And it may well be ever so
slightly quicker.

Signed-off-by: Daniel Stone <daniel@fooishbar.org>