env: fix address violation with '\v' in -S

env: fix address violation with '\v' in -S

Problem reported by Frank Busse (Bug#47412).
* src/env.c (C_ISSPACE_CHARS): New macro.
(shortopts, build_argv, main): Treate all C-locale space
characters like space and tab, for compatibility with FreeBSD.
(validate_split_str, build_argv, parse_split_string):
Use the C locale, not the current locale, to determine whether a
byte is a space character.