Add missing spaces after single-line comments

Add missing spaces after single-line comments

Only 1 of 3 of these changes appear to be handled by pgindent. That change
is new to v15.  The remaining two appear to be left alone by pgindent. The
exact reason for that is not 100% clear to me.  It seems related to the
fact that it's a line that contains *only* a single line comment and no
actual code.  It does not seem worth investigating this in too much
detail.  In any case, these do not conform to our usual practices, so fix
them.

Author: Justin Pryzby
Discussion: https://postgr.es/m/20220411020336.GB26620@telsasoft.com