cp: improve status message when omitting directories
* src/copy.h (cp_options): Add a new flag for install(1).
* src/copy.c (copy_internal): For cp, also output remediation
advice which also indicates why directories aren't copied by default.
The message is unchanged for install(1).
* src/cp.c (cp_option_init): Init install_mode to false.
* src/mv.c (cp_option_init): Likewise.
* src/install.c (cp_option_init): Init install_mode to true.
* tests/install/basic-1.sh: Add a test case.
* tests/cp/link-deref.sh: Adjust test case.
Fixes http://bugs.gnu.org/24958