[flang] Fix usage & catch errors for MAX/MIN with keyword= arguments

Max(), MIN(), and their specific variants are defined with an unlimited
number of dummy arguments named A1=, A2=, &c. whose names are almost never
used in practice but should be allowed for and properly checked for the
usual errors when they do appear.  The intrinsic table's entries otherwise
have fixed numbers of dummy argument definitions, so add some special
case handling in a few spots for MAX/MIN/&c. checking and procedure
characteristics construction.

Differential Revision: https://reviews.llvm.org/D114750