Decoding, validation, and path-resolution fixes for cgi-bin scripts

[b1tw153]

src/cgi-bin/* (17 scripts)

• Pipe the raw parameter values to uncgi on stdin using printf '%s' to ensure that they are correctly decoded.
• Resolve the script's own path and relative paths with realpath to ensure that path construction is correct.
• Validate parameters before use. Data-API scripts return 400 for a malformed request and 500 only for a missing or unreadable backend file. Scripts that render an HTML page fall back to defaults instead of erroring.
• sitemap: fix incorrect handling of an empty directory.
• ping: handle error cases and remove a race condition.
• kill_my_queries: validate the PID argument; construct paths more carefully.
• Quoting, ShellCheck, and backtick → $( ) fixes throughout.

Scripts accept the same inputs and produce the same outputs as before for well-formed requests.

src/pt_diagrams/uncgi.cc

• Properly handle a trailing % at the end of the input.
• Restructure main input loop to ensure the entire input is read correctly.

uncgi accepts the same inputs and produces the same outputs as before for well-formed requests.