patch 9.1.1634: Clipboard code can be improved

patch 9.1.1634: Clipboard code can be improved

Problem:  Clipboard code can be improved
Solution: Slightly refactor code (Foxe Chen).

This commit does the following:
-  Use garray_T when receiving data instead of manually reallocing
-  formatting fixes
-  skip Wayland test that requires clientserver if x11 not compiled
-  Make some functions static

closes: #17999

Signed-off-by: Foxe Chen <chen.foxe@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>