Add more out of memory checks (re: 18529b88) (#192)

The referenced commit neglected to add checks for strdup() calls.
That calls malloc() as well, and is used a lot.

This commit switches to another strategy: it adds wrapper functions
for all the allocation macros that check if the allocation
succeeded, so those checks don't need to be done manually.

src/cmd/ksh93/include/defs.h,
src/cmd/ksh93/sh/init.c:
- Add sh_malloc(), sh_realloc(), sh_calloc(), sh_strdup(),
  sh_memdup() wrapper functions with success checks. Call nospace()
  to error out if allocation fails.
- Update new_of() macro to use sh_malloc().
- Define new sh_newof() macro to replace newof(); it uses
  sh_realloc().

All other changed files:
- Replace the relevant calls with the wrappers.
- Remove now-redundant success checks from 18529b88.
- The ERROR_PANIC error message calls are updated to inclusive-or
  ERROR_SYSTEM into the exit code argument, so libast's error()
  appends the human-readable version of errno in square brackets.
  See src/lib/libast/man/error.3

src/cmd/ksh93/edit/history.c:
- Include "defs.h" to get access to the wrappers even if KSHELL is
  not defined.
- Since we're here, fix a compile error that occurred with KSHELL
  undefined by updating the type definition of hist_fname[] to
  match that of history.h.

src/cmd/ksh93/bltins/enum.c:
- To get access to sh_newof(), include "defs.h" instead of
  <shell.h> (note that "defs.h" includes <shell.h> itself).

src/cmd/ksh93/Mamfile:
- enum.c: depend on defs.h instead of shell.h.
- enum.o: add an -I. flag in the compiler invocation so that defs.h
  can find its subsequent includes.

src/cmd/builtin/pty.c:
- Define one outofmemory() function and call that instead of
  repeating the error message call.
- outofmemory() never returns, so remove superfluous exit handling.

Co-authored-by: Martijn Dekker <martijn@inlv.org>

src/cmd/ksh93/sh/fault.c

@@ -250,11 +250,9 @@ void sh_siginit(void *ptr)
 		tp++;
 	}
 	shp->gd->sigmax = n++;
-	shp->st.trapcom = (char**)calloc(n,sizeof(char*));
-	shp->sigflag = (unsigned char*)calloc(n,1);
-	shp->gd->sigmsg = (char**)calloc(n,sizeof(char*));
-	if(!shp->st.trapcom || !shp->sigflag || !shp->gd->sigmsg)
-		sh_outofmemory();
+	shp->st.trapcom = (char**)sh_calloc(n,sizeof(char*));
+	shp->sigflag = (unsigned char*)sh_calloc(n,1);
+	shp->gd->sigmsg = (char**)sh_calloc(n,sizeof(char*));
 	for(tp=shtab_signals; sig=tp->sh_number; tp++)
 	{
 		n = (sig>>SH_SIGBITS);