build: rework some recipes in man/Makefile.am, for future changes

build: rework some recipes in man/Makefile.am, for future changes

This change is merely required to make future changes easier.

In particular, since we are going to merge the contents of
'man/Makefile.am' into the top-level Makefile, we need to avoid
conflicts with the rules and variables in 'dist-check.mk', and
to prepare for changes in the value of the '$*' variable as used
in the recipe of the '.x -> .1' suffix rule.

* man/Makefile.am (t, mapped_name): Delete, inlining their use ...
(.1.x): ... in the recipe of this suffix rule.  Other adjustments
to prepare to changes in the value of the '$*' automatic variable.
While at it, made more resilient about unlikely but possible failure.
Adjust and reorder few comments.