install: Allow installing stdin

[des]

Summary

If from_name is "/dev/stdin" or "-" and the target is not a directory,
skip the comparison and copy data from standard input to the target.

MFC after: 1 week

[des]

separate devnull and ispipe cases

[imp]

Apart from the comment I don't understand, this is good, I think.

usr.bin/xinstall/xinstall.c

832

comment says no name, but this is can't create a directory, no?

[des]

usr.bin/xinstall/xinstall.c

832

install() has two modes depending on whether the target is an existing file or non-existent path on the one hand, or an existing directory on the other. In the first case, we use the target name as-is, but in the second we create a file in the target directory with the same name as the source. In the devnull and ispipe cases, however, we don't have a valid name to use for the target.

[des]

usr.bin/xinstall/xinstall.c

832

To be clear, install() does not create directories, install_dir() does. What flags & DIRECTORY means is that to_name exists and is a directory.