maint: fix the man page correlation tests

maint: fix the man page correlation tests

These checks were not being run as distcheck-hook targets
are only supported in the top-level Makefile.  Instead
these tests are now run during a syntax-check.

* cfg.mk (sc_man_file_correlation):  A new syntax check to
call the 2 existing tests to check the correlation between
the programs and man/*.[1x].
* man/Makefile.am (sc_man_file_correlation): Call the 2 existing
man page correlation tests.
(check-x-vs-1): Remove the "GEN" annotation as it's a bit verbose.
(check-programs-vs-x): Likewise.
* src/Makefile.am (all_programs.list): Exclude libstdbuf.so
from the list of programs.  This issue was not noticed as
the checks were not actually being run.