env: add -S/--split-string option

env: add -S/--split-string option

Adopted from FreeBSD's env(1), useful for specifing multiple
parameters on a shebang (#!) script line, e.g:

   #!/usr/bin/env -S perl -w -T

Discussed in https://lists.gnu.org/r/coreutils/2018-04/msg00011.html

* src/env.c (valid_escape_sequence,escape_char,scan_varname,
  extract_varname,validate_split_str,build_argv,
  parse_split_string): New functions.
  (main): Process new option and call parse_split_string.
  (usage): Mention new option.
* tests/misc/env-S.pl: Test new option from the command line.
* tests/misc/env-S-script.sh: Test new option from shebang scripts.
* tests/local.mk (all_tests): Add new tests.
* man/env.x (OPTIONS): Show a brief example of -S usage and point to
the full documentation for more information.
* doc/coreutils.texi (env invocation): Detail usage of -S/--split-string
option.
* NEWS: Mention new option.