Compatibility implementation of stpncpy for older systems

[Ryan Schmidt <dash@xxxxxxxxxxxxxx>]

Hello! dash 0.5.12 and the latest code in your git repository fails to build on older versions of macOS (Mac OS X 10.6.x and older). Using its default compiler (Apple's gcc 4.2.1) first this warning appears:

jobs.c: In function 'sprint_status':
jobs.c:434: warning: implicit declaration of function 'stpncpy'
jobs.c:434: warning: incompatible implicit declaration of built-in function 'stpncpy'

Then it fails at link time:

Undefined symbols:
  "_stpncpy", referenced from:
      _sprint_status in jobs.o
ld: symbol(s) not found

Using a newer compiler (llvm.org's clang 11.1.0) the warning is:

jobs.c:434:7: warning: implicitly declaring library function 'stpncpy' with type 'char *(char *, const char *, unsigned long)' [-Wimplicit-function-declaration]
                s = stpncpy(s, strsignal(st), 32);
                    ^
jobs.c:434:7: note: include the header <string.h> or explicitly provide a declaration for 'stpncpy'

And then a similar failure at link time:

Undefined symbols for architecture x86_64:
  "_stpncpy", referenced from:
      _sprint_status in jobs.o
ld: symbol(s) not found for architecture x86_64

The newer clang suggests including <string.h> because it knows the standard says that stpncpy would be declared there, but it won't help on Mac OS X 10.6.x and earlier because stpncpy was introduced in Mac OS X 10.7.

Would it be reasonable to add a compatibility implementation of stpncpy to dash? This and other changes were proposed here:

https://lore.kernel.org/dash/6cacf608-326e-211a-7d37-164041710980@xxxxxxxx/