patch 9.0.1962: No support for writing extended attributes

Problem:  No support for writing extended attributes
Solution: Add extended attribute support for linux

It's been a long standing issue, that if you write a file with extended
attributes and backupcopy is set to no, the file will loose the extended
attributes.

So this patch adds support for retrieving the extended attributes and
copying it to the new file. It currently only works on linux, mainly
because I don't know the different APIs for other systems (BSD, MacOSX and
Solaris).  On linux, this should be supported since Kernel 2.4 or
something, so this should be pretty safe to use now.

Enable the extended attribute support with normal builds.

I also added it explicitly to the :version output as well as make it
able to check using `:echo has("xattr")`, to have users easily check
that this is available.

In contrast to the similar support for SELINUX and SMACK support (which
also internally uses extended attributes), I have made this a FEAT_XATTR
define, instead of the similar HAVE_XATTR.

Add a test and change CI to include relevant packages so that CI can
test that extended attributes are correctly written.

closes: #306
closes: #13203

Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/proto/os_unix.pro b/src/proto/os_unix.pro
index c3a8483..6e13de6 100644
--- a/src/proto/os_unix.pro
+++ b/src/proto/os_unix.pro
@@ -37,6 +37,7 @@
 int mch_setperm(char_u *name, long perm);
 int mch_fsetperm(int fd, long perm);
 void mch_copy_sec(char_u *from_file, char_u *to_file);
+void mch_copy_xattr(char_u *from_file, char_u *to_file);
 vim_acl_T mch_get_acl(char_u *fname);
 void mch_set_acl(char_u *fname, vim_acl_T aclent);
 void mch_free_acl(vim_acl_T aclent);