Handle upsmon NOTIFYCMD and upssched CMDSCRIPT values strictly as paths

[jimklimov]

Documentation was vague about this, and implementation used system() which modern guidelines frown upon. Fix this to mean a single token with an exact path name (or a program in PATH) without arguments, inline scriptlets, etc.

NOTE: SHUTDOWNCMD should not be impacted by this change, and can still take arguments as is often does (e.g. shutdown -t now).

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A ZIP file with standard source tarball and another tarball with pre-built docs for commit 3f1da37 is temporarily available: NUT-tarballs-PR-3499.zip.

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❌ Build nut 2.8.5.4856-master failed (commit c5afe1a9f4 by @jimklimov)

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✅ Build nut 2.8.5.4858-master completed (commit 56e169d096 by @jimklimov)

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[jimklimov]

UPDATE: Upon practical review, e.g. per examples in issue #3525 and PR #3527, there may be significant usage of multi-token command lines, such as the interpreter program name (cmd.exe or powershell.exe on Windows) separate from the script path name, and maybe some further parameters, e.g.:

CMDSCRIPT "C:\\Windows\\System32\\WindowsPowerShell\\v1.0\\powershell.exe -NoProfile -ExecutionPolicy Bypass -File C:\\NUT\\cmd\\upssched-cmd.ps1"

NOTIFYCMD "C:\\NUT\\sbin\\upssched.exe -DDDDD"

# Not directly impacted by this PR as it gets no further args and safely goes still to `system()`
SHUTDOWNCMD "C:\\Windows\\System32\\shutdown.exe /s /t 0 /f"

This PR would break such use-cases, and should be extended to pass through multiple tokens seen by NUT "parseconf" code as separate argv[] entries to execve or CreateProcess calls, e.g.:

CMDSCRIPT "C:\\Windows\\System32\\WindowsPowerShell\\v1.0\\powershell.exe" "-NoProfile" "-ExecutionPolicy" "Bypass" "-File" "C:\\NUT\\cmd\\upssched-cmd.ps1"

NOTIFYCMD "C:\\NUT\\sbin\\upssched.exe" "-DDDDD"

# For good measure, still worth understanding this spelling (even if not requiring it)...
SHUTDOWNCMD "C:\\Windows\\System32\\shutdown.exe" "/s" "/t" "0" "/f"

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✅ Build nut 2.8.5.4927-master completed (commit f4a419731e by @jimklimov)

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✅ Build nut 2.8.5.4931-master completed (commit 6a2b4569c3 by @jimklimov)

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