[ng] uninstall: refactor, more processing delegated to GNU make

[ng] uninstall: refactor, more processing delegated to GNU make

* lib/am/header-vars.mk (am.chars.space, am.chars.tab): New variables,
defined respectively to a literal white space characters and a literal
tabulation character.
(am.util.whitespace-escape): New function, backslash-escape whitespace
characters in the given string.
(am.util.file-exists): New function, tell whether the given argument
is an existing file; it does so coping correctly with whitespaces and
wildcard metacharacters as well, to be usable with paths containing
references to $(DESTDIR).
(am.uninst.cmd.aux): Re-implement using the new $(am.util.file-exists)
function.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>