Fix a rather sizable number of problems in our homegrown snprintf, such as

incorrect implementation of argument reordering, arbitrary limit of output
size for sprintf and fprintf, willingness to access more bytes than "%.Ns"
specification allows, wrong formatting of LONGLONG_MIN, various field-padding
bugs and omissions.  I believe it now accurately implements a subset of
the Single Unix Spec requirements (remaining unimplemented features are
documented, too).  Bruce Momjian and Tom Lane.