Fixup various StringInfo function usages

Fixup various StringInfo function usages

This adjusts various appendStringInfo* function calls to use a more
appropriate and efficient function with the same behavior.  For example,
use appendStringInfoChar() when appending a single character rather than
appendStringInfo() and appendStringInfoString() when no formatting is
required rather than using appendStringInfo().

All adjustments made here are in code that's new to v17, so it makes
sense to fix these now rather than wait a few years and make
backpatching harder.

Discussion: https://postgr.es/m/CAApHDvojY2UvMiO+9_55ArTj10P1LBNJyyoGB+C65BLDNT0GsQ@mail.gmail.com
Reviewed-by: Nathan Bossart, Tom Lane