mkuzip: drop support for executable uzip images
A uzip image has a 128-byte header, historically, this header could be
executed as a shell script to mount the uzip image to a user provided
mountpoint.
The embedded shell commands only work for uzip images that were created
with zlib or zstd compression that contained an ISO-9660 file system.
Given the limited space available in the uzip header, it is not
practical to extend this feature to include other file systems or to
provide sensible error handling and error messages to the user.
For these reasons, abandon the embedded shell script in the uzip image
header.
To maintain backwards compatibility, the shebang and shell must reside
in the 128-byte header.
This change of behavior is documented in mkuzip(8) and an example
has been provided for creating/mounting uzip images.
PR: 276174