tools: jobserver: Prevent deadlock caused by incorrect jobserver configuration and enhance error reporting

When using GNU Make's jobserver feature in kernel builds, a bug in MAKEFLAGS
propagation caused "--jobserver-auth=r,w" to reference an unintended file
descriptor. This led to infinite loops in jobserver-exec's os.read() calls
due to empty token.

My shell opened /etc/passwd for some reason without closing it, and as a
result, all child processes inherited this fd 3.

$ ls -l /proc/self/fd
total 0
lrwx------ 1 changbin changbin 64 Dec 25 13:03 0 -> /dev/pts/1
lrwx------ 1 changbin changbin 64 Dec 25 13:03 1 -> /dev/pts/1
lrwx------ 1 changbin changbin 64 Dec 25 13:03 2 -> /dev/pts/1
lr-x------ 1 changbin changbin 64 Dec 25 13:03 3 -> /etc/passwd
lr-x------ 1 changbin changbin 64 Dec 25 13:03 4 -> /proc/1421383/fd

In this case, the `make` should open a new file descriptor for jobserver
control, but clearly, it did not do so and instead still passed fd 3 as
"--jobserver-auth=3,4" in MAKEFLAGS. (The version of my gnu make is 4.3)

This update ensures robustness against invalid jobserver configurations,
even when `make` incorrectly pass non-pipe file descriptors.
* Rejecting empty reads to prevent infinite loops on EOF.
* Clearing `self.jobs` to avoid writing to incorrect files if invalid tokens
are detected.
* Printing detailed error messages to stderr to inform the user.

Cc: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Reviewed-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Changbin Du <changbin.du@huawei.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <20260108113836.2976527-1-changbin.du@huawei.com>