MINOR: tree-wide: add missing TAINTED flags for some experimental directives

MINOR: tree-wide: add missing TAINTED flags for some experimental directives

We normally taint the process when using experimental directives, but
a handful of places were missed so we don't always know that they are
in use. Let's fix these places (hint for future directives, just look
for places checking for "experimental_directives_allowed", and add
"mark_tainted(TAINTED_CONFIG_EXP_KW_DECLARED);").