API: Warn when unsupported PHP array syntax is used

The API takes multi-valued parameters as key=value1|value2|value3, not
key[]=value1&key[]=value2&key[]=value3, in part because the latter is
overly verbose when the API encourages use of large arrays.

But when someone, not knowing this, does accidentally use the verbose
syntax, they should get a warning instead of having the parameter be
silently ignored.

Bug: 64057
Change-Id: I32a16efb8028d7f6d120d20dfc886f08ed9ec97d
Merge "API: Warn when unsupported PHP array syntax is used"