doc: fix inconsistent capitalization in --help output
* src/base64.c (usage): Don't capitalize the first character
in an --option description.
* src/stdbuf.c (usage): Likewise.
* src/truncate.c (usage): Likewise.
* cfg.mk (sc_option_desc_uppercase): A new syntax check to
stop this happening in future.
* man/Makefile.am (sc_option_desc_uppercase): Ensure all
man pages are generated, then search for erroneous uppercase chars.
* src/Makefile.am (all_programs): Ensure all
commands are built so that all man pages can be generated.