bug#72707: install: need_copy (for "-C") should use stat instead of lstat

[Frank Heckenbach]

install dereferences symlinks given as sources (and I think that's
good). The "-C" option, however, uses lstat rather than
stat in need_copy and returns true if !S_ISREG.

So the (dereferenced) file will always be copied despite "-C".

Even worse, since "-p" cannot be combined with "-C", the installed
file will always get a new timestamp which may trigger rebuilds
(e.g. if the file is a header) even if nothing was changed, e.g.:

% touch a
% ln -s a b
% install -C b c
% ls --time-style=+%T -l
total 0
-rw------- 1 frank frank 0 23:46:51 a
lrwxrwxrwx 1 frank frank 1 23:46:54 b -> a
-rwxr-xr-x 1 frank frank 0 23:46:58 c
% install -C b c
% ls --time-style=+%T -l
total 0
-rw------- 1 frank frank 0 23:46:51 a
lrwxrwxrwx 1 frank frank 1 23:46:54 b -> a
-rwxr-xr-x 1 frank frank 0 23:47:06 c
% install -C b c
% ls --time-style=+%T -l
total 0
-rw------- 1 frank frank 0 23:46:51 a
lrwxrwxrwx 1 frank frank 1 23:46:54 b -> a
-rwxr-xr-x 1 frank frank 0 23:47:08 c
%

I asked Kamil Dudka who implemented "-C" originally and he pointed
out that requiring both the files to be regular files was suggested
by Jim Meyering in the review:
https://lists.gnu.org/archive/html/bug-coreutils/2009-02/msg00106.html

However, I don't see a real reason there, only his question:
"Have you considered requiring that both files be `regular', too?"

I don't know if install didn't dereference symlinks back then, but
now that it does, it seems consistent to dereference them in the
check as well, i.e. use stat rather than lstat.