valid_field(): Improve readability

[alejandro-colomar]

This adds strisprint() and strchriscntrl().

---

Revisions:

v2

• Rewrite the for loops in strisprint() and strchriscntrl().
• Add comment desoupifying API names.

$ git range-diff shadow/master gh/vf vf 
1:  8561e822 ! 1:  9d2d0f66 lib/string/ctype/strisascii/: strisprint(): Add function
    @@ lib/string/ctype/strisascii/strisprint.c (new)
     
      ## lib/string/ctype/strisascii/strisprint.h (new) ##
     @@
    -+// SPDX-FileCopyrightText: 2024, Alejandro Colomar <[email protected]>
    ++// SPDX-FileCopyrightText: 2024-2025, Alejandro Colomar <[email protected]>
     +// SPDX-License-Identifier: BSD-3-Clause
     +
     +
    @@ lib/string/ctype/strisascii/strisprint.h (new)
     +  if (streq(s, ""))
     +          return false;
     +
    -+  for (unsigned char c = *s; !streq(s, ""); c = *s++) {
    ++  for (; !streq(s, ""); s++) {
    ++          unsigned char  c = *s;
    ++
     +          if (!isprint(c))
     +                  return false;
     +  }
2:  e0404e20 ! 2:  83feb72a lib/string/ctype/strchrisascii/: strchriscntrl(): Add function
    @@ lib/string/ctype/strchrisascii/strchriscntrl.c (new)
     
      ## lib/string/ctype/strchrisascii/strchriscntrl.h (new) ##
     @@
    -+// SPDX-FileCopyrightText: 2024, Alejandro Colomar <[email protected]>
    ++// SPDX-FileCopyrightText: 2024-2025, Alejandro Colomar <[email protected]>
     +// SPDX-License-Identifier: BSD-3-Clause
     +
     +
    @@ lib/string/ctype/strchrisascii/strchriscntrl.h (new)
     +inline bool strchriscntrl(const char *s);
     +
     +
    ++// string character is [:cntrl:]
     +// Return true if any iscntrl(3) character is found in the string.
     +inline bool
     +strchriscntrl(const char *s)
     +{
    -+  for (unsigned char c = *s; !streq(s, ""); c = *s++) {
    ++  for (; !streq(s, ""); s++) {
    ++          unsigned char  c = *s;
    ++
     +          if (iscntrl(c))
     +                  return true;
     +  }
3:  3bf69d20 = 3:  cb7ac416 lib/fields.c: valid_field(): Remove useless check
4:  dd9dcf99 = 4:  2633c872 lib/fields.c: valid_field(): Return early on error
5:  8c894903 = 5:  40bdb529 lib/fields.c: valid_field(): Use strisprint() instead of its pattern
6:  179652c2 = 6:  a47c6de7 lib/fields.c: valid_field(): Use strchriscntrl() instead of its pattern
7:  8032894a = 7:  fdee80fc lib/fields.c: valid_field(): Clarify comments

v2b

• Rebase

$ git range-diff master..gh/vf shadow/master..vf 
1:  9d2d0f66 = 1:  793e5089 lib/string/ctype/strisascii/: strisprint(): Add function
2:  83feb72a = 2:  3cd6275c lib/string/ctype/strchrisascii/: strchriscntrl(): Add function
3:  cb7ac416 = 3:  ad633f26 lib/fields.c: valid_field(): Remove useless check
4:  2633c872 = 4:  4fe506c8 lib/fields.c: valid_field(): Return early on error
5:  40bdb529 = 5:  2ebb6672 lib/fields.c: valid_field(): Use strisprint() instead of its pattern
6:  a47c6de7 = 6:  316c8984 lib/fields.c: valid_field(): Use strchriscntrl() instead of its pattern
7:  fdee80fc = 7:  819f6ca0 lib/fields.c: valid_field(): Clarify comments