maint: avoid file-scope names of the form _[a-z]*

maint: avoid file-scope names of the form _[a-z]*

The C standard says this isn't portable, if you include
standard include files.
* build-aux/gen-single-binary.sh:
* src/coreutils-arch.c (single_binary_main_arch)
(single_binary_main_uname):
* src/coreutils-dir.c (single_binary_main_ls)
(_single_binary_main_dir):
* src/coreutils-vdir.c (single_binary_main_ls)
(_single_binary_main_vdir):
* src/coreutils.c (SINGLE_BINARY_PROGRAM):
Remove leading _ from single_binary prefix.
* src/numfmt.c (round_style): Rename from _round.  All uses changed.
(inval_style): Rename from _invalid.  All uses changed.