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.

AUTHORS

@@ -26,7 +26,7 @@ dircolors: H. Peter Anvin
 dirname: David MacKenzie, Jim Meyering
 du: Torbjorn Granlund, David MacKenzie, Paul Eggert, Jim Meyering
 echo: Brian Fox, Chet Ramey
-env: Richard Mlynarik, David MacKenzie
+env: Richard Mlynarik, David MacKenzie, Assaf Gordon
 expand: David MacKenzie
 expr: Mike Parker, James Youngman, Paul Eggert
 factor: Paul Rubin, Torbjörn Granlund, Niels Möller

NEWS

@@ -53,6 +53,10 @@ GNU coreutils NEWS                                    -*- outline -*-
   env supports a new -v/--debug option to show verbose information about
   each processing step.
 
+  env supports a new -S/--split-string=S option to split a single argument
+  string into multiple arguments. Used to pass multiple arguments in scripts
+  (shebang lines).
+
   md5sum accepts a new option: --zero (-z) to delimit the output lines with a
   NUL instead of a newline character.  This also disables file name escaping.
   This also applies to sha*sum and b2sum.