maint: convert .x-sc_* into exclude_file_name_regexp--sc_* exemptions

maint: convert .x-sc_* into exclude_file_name_regexp--sc_* exemptions

Many of the .x-sc_* exemptions were no long necessary.  Remove those
files and instead, provide exemptions via variable definitions in
cfg.mk to address the few remaining exceptions.
* .x-sc_prohibit_atoi_atof: Remove file.
* .x-sc_space_tab: Likewise.
* .x-sc_sun_os_names: Likewise.
* .x-sc_trailing_blank: Likewise.
* .x-sc_two_space_separator_in_usage: Likewise.
* .x-sc_useless_cpp_parens: Likewise.
* cfg.mk: Add minimal exemptions.
* cfg.mk: Add minimal exemptions.
* doc/standards.texi (Standard C): Address the sole useless-cpp-parens
violation in this file:
-#if defined (__STDC__) || defined (WINDOWSNT)
+#if defined __STDC__ || defined WINDOWSNT
With that, the only remaining offender is config.guess, whose name
is now listed in cfg.mk.
Suggested by Eric Blake.