Avoid non-POSIX cp flags

Commit 252dcb32 used cp -a, but apparently Solaris doesn't like that.  Use cp
-RPp instead.

Author: Thomas Munro <thomas.munro@gmail.com>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Discussion: https://postgr.es/m/CA+hUKGL10AoQVMMqgOJ8CTjoz9MLidD8ik2e8PibzLNMz0+aRg@mail.gmail.com

src/test/perl/PostgreSQL/Test/Cluster.pm

@@ -549,7 +549,7 @@ sub init
 		}
 		else
 		{
-			@copycmd = qw(cp -a);
+			@copycmd = qw(cp -RPp);
 			$expected_exitcode = 0;
 		}
 

src/test/regress/pg_regress.c

@@ -2355,7 +2355,7 @@ regression_main(int argc, char *argv[],
 		else
 		{
 #ifndef WIN32
-			const char *copycmd = "cp -a \"%s\" \"%s/data\"";
+			const char *copycmd = "cp -RPp \"%s\" \"%s/data\"";
 			int			expected_exitcode = 0;
 #else
 			const char *copycmd = "robocopy /E /NJS /NJH /NFL /NDL /NP \"%s\" \"%s/data\"";