Protect processing of custom scripts in glidein_startup against stdin interference

[mambelli]

As reported in Issue #500 , on a specific site the parsing of the client custom scripts list was stopping after a specific file and continuing with the next list.
This PR fixes #500 :

• redirects the stdin of custom scripts to /dev/null to avoid to interfere with glidein_startup.sh stdin
• adds a check to report a warning in case of similar problems

The last commit is the fix for production

This PR includes also older commits with modified versions used in troubleshooting (debug messages). To keep them in the history.
Consider a squash-merge to exclude them from the master branch history.

[mambelli]

To help troubleshoot:

1. replace glidein_startup.sh in /var/lib/gwms-factory/web-base/ with the copy in this PR (direct link to the raw file: https://raw.githubusercontent.com/glideinWMS/glideinwms/09cfe1a9db2b0ac4decb9b992334c7b554384d32/creation/web_base/glidein_startup.sh).
2. Then run a factory upgrade (reconfig is insufficient)

This will add about 20+2*N_of_downloads lines to the glidein stderr

[mambelli]

New version https://raw.githubusercontent.com/glideinWMS/glideinwms/d01e1b2519e8a41629b08a423ab6442ff749b826/creation/web_base/glidein_startup.sh

[mambelli]

New version (3). Added a wrapper in the read loop parsing the files to get more troubleshooting info: https://raw.githubusercontent.com/glideinWMS/glideinwms/9642ae5dc1828ddd04b677f40611fc2d8f2f1992/creation/web_base/glidein_startup.sh

[mambelli]

This fix will go in GlideinWMS (Factory) v 3.10.11 and v 3.11.1
To apply this fix you can patch any Factory (no changes for the Frontend) v 3.10.10 and 3.11.0.

1. replace glidein_startup.sh in /var/lib/gwms-factory/web-base/ with the copy in this PR (direct link: https://raw.githubusercontent.com/mambelli/glideinwms/refs/heads/v310/file_fetch_troubleshoot/creation/web_base/glidein_startup.sh) and
2. run a factory upgrade (reconfig is insufficient)

[namrathaurs]

Changes look good and can be merged!