library: correct 'vectorized' string logic, <PIDS> api

The commit referenced below addressed (some) anomalies
surrounding 'strv' pointers. However, there remained a
couple quirks involving a potential NULL return value.

Any NULL values returned from the old library readproc
guys would cause no real harm for newlib. But they did
produce the misleading "[ duplicate ENUM_ID ]" result.

The following all represent potential NULL results and
suggest shortcomings in testing of that earlier patch.

. kernel threads do not have cgroup, cmdline & environ
. even if present environ could require root to access

So, this patch reverts a portion of the earlier commit
and ensures when some vectored string is not available
a traditional dash ('-') is the 'strv' returned value.

[ and we'll also correct one typo in the header file ]

Reference(s):
. eliminated a final potential NULL, <PIDS> api
commit 09503dc597

Signed-off-by: Jim Warner <james.warner@comcast.net>

proc/readproc.h

@@ -260,5 +260,6 @@ proc_t* readproc(PROCTAB *__restrict const PT, proc_t *__restrict p);
 proc_t* readeither(PROCTAB *__restrict const PT, proc_t *__restrict x);
 int look_up_our_self(proc_t *p);
 void closeproc(PROCTAB* PT);
+char** vectorize_this_str(const char* src);
 
 #endif